The first generation iPad Air, iPad mini2 and mini3 can only be updated to iOS 12.5.6.
Apple ended major update support for these models of iPad in September 2019 - although they continue to receive occasional “point” security updates, the most recent being released August 2022. These devices cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements (CPU capabilities and available RAM) for newer 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 your iPad, 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 iPadOS15. Given that your device is within its twilight period of update support, if used with financial and e-commerce Apps, you might be well advised to soon consider replacement with a more recent device - in so doing benefiting from both enhanced functionality and full update support.
Sorry, I appreciate that this may not be what you were hoping to learn.
I hope this information provides helpful clarity as to your available options - and the fundamental limitations of your existing iPad.
