If your AppleID and Password is not accepted, this would suggest that the credentials are perhaps incorrect.
If you are certain that the credentials are good (test them at https://appleid.apple.com), then try a forced restart of your iPad - and then try again.
To force-restart your iPad:
- On an iPad with a Home button: Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
- On an iPad with Face ID: Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the top button. When the Apple logo appears, release the button.
If, after restart, you are still unable to turn-off ScreenTime, you are running out of options - and it may become necessary to reset your iPad and set-up as new. Before doing so, I suggest reaching-out to Apple Support.
The Contact Support link can be found at top-right of this page - or, alternatively, you can initiate a support call using the Apple Support App:
https://apps.apple.com/gb/app/apple-support/id1130498044
Please let us known how you get on.