When you try to add a Gmail account to your Outlook (classic) profile, you may get the following error. This will occur before entering your account information or password.

To resolve this issue follow these steps:
1. Open Control Panel > User Accounts > Credential Manager > Manage Windows Credentials
2. Delete any MicrosoftOffice16_DATA:OAUTH2 entries
3. Close and restart Outlook
4. Attempt to re-add the Gmail account, you should now be redirected to a web login for Gmail. If you are not automatically redirected, check off manual setup and choose the Google option.