I have done everything on my iPad to start split View but it won’t work. I tried to do it using assistive touch but noit says that unable to put application in split View. I have an iPad Air, I don’t know which generation it is, just the simple iPad.

A first generation iPad Air can only be updated to iOS 12.4.9.

Apple ended update major support for iPad Air1 in September 2019, however, the Air1 has continued to receive “point” security updates since then. 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 new 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 the most recent versions of iOS, 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.

You own a 2013, 1st generation iPad Air.

That year iPad Air cannot ever be upgraded beyond any versions of iOS 12, which is currently at iOS 12.4.9.

Also, the 2013, 1st gen iPad Air is NOT capable of using 50/50 split screen view multitasking.

That iPad model can only use the narrower Slide Over screen/view.

That iPad's internal hardware is too underpowered to use to 50/50 split screen view, as well as NOT powerful enough to upgrade to be able to use any of the new and advanced features that are an integral part of the newer iPadOS versions.

Sorry.