iOS 15 update

If you have the original iPad Air iOS 12 is the latest available main update.

iOS and iPadOS devices listed by the last compatible OS

If you need the newer iPadOS versions, you'll need to retire your obsolete iPad Air if you have the original air and it is running iOS 12.5.x. The first air was released in 2013, 9 year ago. It is too old, too underpowered and lacks the hardware required to run anything higher. Time to invest in a newer, more modern iPad at this point.