Unless the App subscription is being purchased directly from the App Developer's own website, the App Subscription is most likely being activated as an in-App Purchase via the Apple App Store.
Assuming that your iPad is not centrally managed by an Educational Establishment or Business (as the device may be remotely managed) - if you are experiencing difficulties with adding, updating or removing Apps from your iPad, you’ll need to check your ScreenTime restrictions:
- Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Adding Apps - set to Allow
- Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Deleting Apps - set to Allow
- Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > In-app Purchases - set to Allow