Can't save anything on iPad

With the Files App open, tap more (“…”) button at top-right of the sidebar - to expose a menu. From the menu, select Edit Sidebar.

Within the list of Locations, ensure that you have enabled On My iPad.

Tap Done when you’re finished.

It seems that the much earlier version of Files for iOS12 differs from iPadOS 13/14/15/16. The More button, present at top-right of the sidebar in all versions of iPadOS, is an Edit button for iOS12. However, the Edit button should reveal a similar Locations selector.

As your iPad is not functioning correctly, you might try a forced restart - as this will often clear an isolated software issue:

Force restart iPad (models with the Home button)

Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.

If you continue to experience difficulties, you would be best advised to seek assistance directly from from Apple Support. You can contact the Support Team using the Support link at top-right or bottom-left of this page.

Alternatively, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:

https://apps.apple.com/app/apple-support/id1130498044

The Select button is at top-right of the Files App window.

The sidebar - at the left of the screen - is where you will find the More (“…”) button - and the associated action menu.