Your iPad won’t be at iOS12.8 - as the version doesn’t exist. Perhaps your iPad is more likely running iOS 12.4.8?
If so, then I surmise that you have an original first generation iPad Air, an iPad mini2 or mini3 - as only these models of iPad can be updated to iOS 12.4.8.
Apple ended update support for these devices in September 2019. They cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS. Since ending major-update support, these models have benefited from security “point” updates over the last year.
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.
I hope this information provides helpful clarity as to your available options - and the limitations of your existing iPad.
