Not able to upgrade the IOS to 13 version

The first generation iPad Air, iPad mini2 and mini3 can only be updated to iOS 12.5.4.

Apple ended major update support for these models of iPad in September 2019 - although they since continue to receive “point” security updates - the most recent being released just a few weeks go. 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 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, 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 iPadOS14. However, for Netflix, if you have a newer device with which to initially download the Netflix App, you will subsequently be able to download an older compatible version of the App to your iPad Air1.

I hope this information provides helpful clarity as to your available options - and the limitations of your existing iPad.

The original iPad Air, released by Apple in 2013, cannot support an iOS version greater than 12.X. A newer iPad is required.