Are cookies accepted in Edit/Preferences/Privacy & Security? That's needed for OAuth authentication. Turn off a VPN if you have one. The outgoing server is smtp.gmail.com, not smpt.gmail.com.

Unfortunately it didn't, Cookies are accepted and retained until they expire. SMTP typo was corrected but didn't make any difference. I have no VPN.

Problem is authenticating with IMAP server.

IMAP for gmail is usually enabled by default in gmail settings, so it's hard to explain why it's not working. Where does it fail? Does the OAuth window appear?

The account is set up in TB and it fails when you launch TB (as it attempts to log in automatically). It also gives the same error message when you click on "Get Messages".

No OAuth window or message shows up.

If you remove all gmail entries from Saved Passwords in Preferences/Privacy & Security, then restart TB, you should at least be shown the OAuth window and be prompted to enter the account password.

No OAuth window opened. Instead, a login to Google Accounts opened and wouldn't accept the password.

However, I went back to the webmail and looked more closely at it and found that the email address in the webmail account was off by one character from the address my brother-in-law told me. (It's his computer I'm trying to set up for him.) When I changed the email address to the one matching the webmail account, it logged right in.

Problem solved.