My iPad Air 2 doesn’t let me update it to iOS 15.7.9 even though it is compatible and I have enough storage what do I do?

I found my brothers old iPad Air 2 and logged into my Apple ID everything seemed to be fine but when I tried to download a certain app that needed iOS 14 or later and I checked what iOS my iPad was running on and it was 13.3 I think. I went to try to update it and I saw that iOS 15.7.9 was available but whenever I tried to install it it told me that an error had occurred even though I made sure I had enough storage and that it was compatible with my iPad