I wish to downgrade my plan, which renewal is due tomorrow, but was unable to click "done". Why is that so?

This is primarily a user-to-user support forum; other Apple users like you. See the following from Downgrade or cancel your iCloud storage 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.