A first generation iPad Air can only be updated to iOS 12.5.1.
Apple ended major update support for iPad Air1 in September 2019 - although it since continues to received “point” security updates. 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 (CPU and 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 Air1, if you want or need to access to Apps requiring a more recent version of iOS than your iPad can support (such as Netflix), you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS14.
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 limitations of your existing iPad Air.