Miscellaneous topics that do not fit into specific categories.
Have you checked for any updates to the app &/or phone?
Can you add the account to the Apple email app to see if that works?
Are you running any Adblocker app or VPN?
If entering the email address & account password doesn’t take you to Google to verify/authorising adding the account then you may require an app specific password which is used instead of the account password.