As described, you are attempting to install an App that requires a higher-version of iOS/iPadOS than you iPad can support. Your iPad, however, is an older model that is already at its highest supported version of iOS.
Whether or not you are able to further update your iPad to a newer version of iOS/iPadOS will be dictated by your iPad model - and the currently installed version of iOS. You can find this information here:
Settings > General > About
Not sure which iPad you have? Identify your iPad model.
Here is a list of the highest version of iOS supported by the older models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated. Per your description, you are almost certain to have one of these models:
iPad1 - iOS 5.1.1
iPad2, iPad3 and iPad mini1 - iOS 9.3.5 (WiFi Only models), iOS 9.3.6 (WiFi & Cellular)
iPad4 - iOS 10.3.3 (WiFi Only), iOS 10.3.4 (WiFi & Cellular)
iPad Air1, iPad mini2 and iPad mini3 - iOS 12.5.2
Newer models, not appearing in this list, are all able to be updated to the current version, iPadOS 14.4.2.
Many 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.
Sorry, I appreciate that this may not be what you were hoping to learn.
I hope this information provides helpful clarity as to the potential limitations of your existing iPad
