Using Classic Outlook on Windows for personal email, calendar, and contact management
There is a problem with Outlook this morning which is giving users a 500 error code but just in case this isn’t the cause in your case when you’re asked to authenticate are you taken to a Google sign in page in order to do this or does the error occur at that time?
When browser are you using? If you change the default browser to a non Chrome related one does the same error occur? If yes then it may be the temporary glitch Microsoft is experiencing.
Outlook classic isn’t compatible with the new OAUTH 2.0 security protocol so you either will need to upgrade to a newer Outlook, create & use an app specific password (less secure though) & enter this instead of the account password or use a more modern email client like Outlook new or Firefox.