Why Apple Ipad Air 2 cannot update anymore? I want to Install netflix but need iOS update which I cannot update

Are you certain of your iPad model - and its capability of being updated to iPadOS14.x? Details of your iPad model and currently installed version of iOS can be found here:

Settings > General > About

Not sure which iPad you have? Identify your iPad model.

It is not unusual for a first generation iPad Air to be misidentified and an Air2.

A first generation iPad Air can only be updated to iOS 12.5. If instead you have an iPad Air2, or the current Air3, these do support the current version - iPadOS14.3.

Apple ended update support for iPad Air1 in September 2019. An iPad Air1 cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS.

Some App developers have dropped support for iOS versions preceding iOS 13.x for technical reasons (often because APIs or software libraries upon which they rely have been updated in the new version of iPadOS, making their App incompatible with earlier versions). 

Whilst many App titles remain compatible with an iPad Air1, if you want or need to access to Apps requiring the most recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS14.

If you do have an Air2 or Air3, it should be possible to simply update your iPad, without a PC/Mac, from Settings:

Settings > General > Software Update

Sorry, I appreciate that this may not be what you were hoping to learn if you do, indeed, possess an iPad Air1.

I hope this information provides helpful clarity as to your available options - and the limitations of your existing iPad Air.

if it's a ipad air 1g  iOS 12.5,  is last ios it support

if it's an ipad air 2g then read Update your iPhone, iPad, or iPod touch - Apple Support