The last drastic measure to try ( you WILL NEED access to a Mac or Windows PC to do these next steps!
Make sure your iPad is, at least, charged to 50% or greater to attempt this.
Do a backup of your iPad's data, prior to upgrading/updating.
Backup to Apple iCloud and/or a full Mac or Windows PC using the same iTunes app OR, if a newer Mac running macOS Catalina, or later, making a backup using the macOS Finder app.
How to back up your iPhone, iPad, and iPod touch - Apple Support
Then, Go to the Settings app and under the General settings panel, tap Reset, then Erase ALL Contents and Settings to completly erasse your iPad and set this iPad up as a brand new iPad from scratch.
At this point, DO NOT DO ANY RESTORE of YOUR DATA, yet!
Having access to a full computer, like a Mac or Windows PC, try either using the Finder on a Mac that has the newest macOS Catalina, or later macOS, installed OR with an older Mac with an older Mac OS, download and install the latest iTunes version ( or latest iTunes for Windows PC ), make sure the Mac or Windows PC is connected to the Internet, connect your iPad by the USB charge cable to the computer and let the iTunes application upgrade/update that iPad model, using the iTunes Software Update feature, to its current, maximum OS of iPadOS 15.7.
IF the 15.7 upgrade of your 16 GBs iPad Air 2 is successful using a Mac or Windows PC,
then try to restore the backup you made on the computer, back to your iPad.
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
If the restore of data backup is, also, sucxessful, I strongly advise going through that iPad and start removing deleting anything that is NOT really needed on that low storage iPad model.
Thin out/delete any music/videos/movies/TV shows/podcasts/screen recordings/photos-images/eBooks/
eMagazine/iBooks/audio books/Documents/PDFs, etc.
If you use a POP email account on your iPad, delete/thin out any old saved emails that maybe saved to your iPad.
To permanently delete photos/images off of your iDevice go into the Photos app and find the Recently Deleted folder/album and permanently delete any photos/images stored in that folder/album.
Then, for older iPad models with a Home button, do a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons.
If NO access to a Mac or Windows computer, you will have to seek help from a good friend or family member, with a full Mac or Windows PC, to help you out
OR
Search online to locate and call contact your nearest, “local” Apple Store location to try/attempt to make an appointment to get your Apple iPad upgraded/updated to its latest iPadOS 15 version.
OR, alternatively, use the “Get Support” link, at the upper right hand corner of every Apple Support Community Webpage to attempt to schedule an appointment with your nearest Apple Store location.
Apple Store employees will be glad to help/assist you.
Best of Luck to You!