Can’t click done to change storage

See the following from Downgrade or cancel your iCloud+ plan - Apple Support

If you can't click Done, make sure you are signed in with the same Apple ID as your iCloud storage plan. You can also try following these steps on a different device. If you still need help, contact Apple Support.

Did you read the article at the top left of this thread & follow the steps?

Scroll to If you can't select Done.