I cannot connect the 1st generation pen to my iPad Air 2

First, check compatibility…

Apple Pencil compatibility - Apple Support

Assuming that you have an Apple Pencil that is compatible with your iPad, this support page may be helpful with pairing and use of the Pencil with your iPad:

Connect Apple Pencil with your iPad - Apple Support

If you continue to experience difficulties, here is one of the better troubleshooting guides:

https://appletoolbox.com/apple-pencil-not-working-heres-our-troubleshooting-guide/

Be aware that if your Pencil has been set-aside for an extended period, unused and uncharged, the Pencil battery may now be irretrievably damaged through deep discharge. In common with all Li-ion batteries, they must always have some level of charge.

The tiny internal Li-ion battery is susceptible to permanent/irreversible damage through being left discharged for long periods. Even some “new” pencils can exhibit signs of failure out-of-the-box if they are “old stock”. 

It is essential that if you have an Apple Pencil that you charge it regularly - whether used or not - so as to protect the battery from deep-discharge. Do not allow a pencil to remain in low-charge state for any period of time - as the internal battery will fail, rendering the Pencil useless. Setting aside an unused Pencil, for extended periods, is a recipe for premature death of the Pencil battery.

If the Pencil Battery has failed, the only remedy is to replace the Pencil. If the battery has failed and your pencil is within its one-year warranty, you should look to having it replaced by your retailer or at an Apple Store.

Some reports suggest that gentle warming of the Pencil (such as a hot water radiator) prior to attempting charging of the Pencil can resurrect a Pencil battery. Whilst I do not advocate external heating of any Li-ion battery (this being potentially very dangerous), gentle external warming of a tiny battery is unlikely to lead to catastrophic failure. 

Please see the compatibility list at: https://www.apple.com/shop/select-apple-pencil

No Pencils work with any old/er 6-10-year old iPad models, as well as the 2017, iPad 5th generation.

These old/er iPad models lack the crucial and necessary internal screen hardware layer needed for an Pencil to be recognised by an iPad's screen and to properly function/interact with an iPad’s screen.

The ONLY third party, smart Bluetooth stylus that I can, honestly, recommend that actually works well with any older iPad, that is NOT compatible with any Pencils, is the Adonit smart Bluetooth stylus.

https://www.adonit.net/jot/pixel/

This smart, Bluetooth stylus works across a wide range of iOS devices.

The Bluetooth connection is stable and its Bluetooth pressure sensing technology is very smooth, natural feeling and has consistent line drawing ability.

Plus it's a quality made, aluminum stylus with two programmable buttons.

I had been using this stylus for well over 3 years now and it is/was really good on my iPad Pro.

This stylus worked wonderfully for me with all compatible apps.

The Adonit Pixel works with iPads all the way back to the 2012, 1st gen iPad Mini and iPad 4th generation!

Has ON/OFF/app shortcut buttons.

Has a nice fine, textured pen tip AND a nice rubber grip.

Magnetically charges on its own magnetic charging adapterand has an auto 15 minute shutdown feature ( to preserve battery life ), charges in a hour, or so.

Has both a small USB charger that that plugs into any USB charging block OR you can purchase an optional charging dock.

I find it only lasts for anywhere between 9-12 hours, but Adonit claims up to 16 hours of continuous use on a single charge cycle.

Made of very high quality aluminum materials.

The Adonit Pixel ONLY works with approx. 24 of the major/popular drawing/sketching/painting apps (the Pixel works with, but currently no longer “officially” supported with Procreate, but it still works and works pretty well in Procreate ), and approx. half dozen of the popular note taking apps, a half dozen writing apps and about a half dozen PDF style/compatible apps.

You WILL have to perform some pairing and setup parameters in each of the Adonit Pixel compatible apps.

You just DO NOT simply turn on Bluetooth on an iDevice and turn on the Adonit stylus and start writing.

The Adonit Pixel stylus is NOT an Pencil in this regard/respect.

You have to initially Bluetooth pair the Adonit Pixel, setup the hand/palm position and in many apps, set up the stylus screen pressure sensitivity for each and every compatible app!

Once you have, initially, done all of this, all Adonit Pixel compatible apps will remember these settings for future use!

Some Adonit Pixel users claimed their stylus doesn’t work, but it does work well IF you properly know how to set up the Adonit Pixel!

If interested, to make sure you get the latest installed stylus firmware, you must order the Adonit Pixel stylus directly from the Adonit website, when possible and available.

This stylus has just become available, once again, after about year of absence from availability from Adonit.

So, get this stylus while it is in stock and while supplies last.

Here’s my “up to date”, comprehensive, but still incomplete, list of Adonit Pixel compatible apps for the iPad/iPad Pro.

MOST apps listed support both screen pressure sensing and palm rejection, unless otherwise noted

Compatible Adonit Pixel apps for drawing

Animation Desk

Amaziograph

Adobe Photoshop Sketch ( palm rejection ONLY, with NO screen pressure sensing/line width variations with older iPad Pro models. Palm and screen pressure sensing/variable line width drawing and writing with all other non-Apple Pencil compatible iPad models.

A great app with a lot of free brushes without having to pay monthly to subscribe )

Adobe Draw

ArtRage

Art Studio Pro ( fully supports the Adonit Pixel and is just as great as Procreate on the iPad )

AutoDesk SketchBook

Colored Pencil

Comic Draw

Concepts

Forge ( drawing/sketching/concept/ideation app )

Interactive SketchBook

Medibang Paint

Procreate

(Adonit Pixel has been supported as an “unsupported stylus”, for some time now, but all Adonit Pixel stylus features still work as they should in Procreate, palm rejection IS improved using special, non-body electrical conducting, lycra-based material drawing glove )

Tayasui Sketches/Sketches Pro ( Full support for the Adonit Pixel )

Tayasui Sketches School ( FREE! )

( this new, free version works with this stylus, but without any palm rejection or pressure sensing, so you need to use a special, non-body electrical conducting drawing glove with the Sketches School version to lay your hand/palm down on the iPad’s screen while writing or drawing )

Vectornator

Zen Brush 2

Compatible Adonit Pixel apps for Notes and Notes with drawing types of apps.

GoodNotes

INKredible

Note Shelf 2

Notes Writer ( palm rejection improved using special, non-electrical conducting, lycra-based material drawing glove )

QuickNotes X Pro ( palm rejection ONLY. No pressure sensing for light/dark or thin/thick lines )

uPad

Zoom Notes

PDF Pen

This is pretty much it for Adonit Pixel compatible note-taking apps.

This stylus works with other non-compatible apps, too, but without any palm rejection and/or pressure features.

You can use this stylus with other apps but will not be able to lay your palm on the screen, while you write or draw, unless you use special, non-body electrical conducting, lycra-based material drawing glove that works by isolating your hand/palm and allow you to lay your hand/palm on the screen as you draw in these non-compatible Adonit stylus apps.

Something like these found on Amazon.

https://www.amazon.com/AKX-Rejection-Drawing-Graphic-Sketching/dp/B08R6PWKC9/ref=sr_1_33_sspa?dchild=1&keywords=tablet+drawing+gloves&qid=1611535160&sr=8-33-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUExMFg5WFBZWFpCS1kmZW5jcnlwdGVkSWQ9QTA5OTA2MzMxRUVYUURUMlAwVU4wJmVuY3J5cHRlZEFkSWQ9QTAxNDY1NjMxVjg1NE45TVRXSUQ3JndpZGdldE5hbWU9c3BfYXRmX25leHQmYWN0aW9uPWNsaWNrUmVkaXJlY3QmZG9Ob3RMb2dDbGljaz10cnVl

https://www.amazon.com/Timebetter-Rejection-Drawing-Sketching-Suitable/dp/B08DKGHYMS/ref=sr_1_21?dchild=1&keywords=drawing+gloves&qid=1611533128&sr=8-21

Many more makers of these types of gloves found on Amazon and in different hand sizes, as well!

Best of Luck to You!