Which server settings are you using? Are you performing a manual configuration of Tbird? See https://www.att.com/support/article/email-support/KM1010523/ for the right settings to use.

I tried all the settings in the instructions and none of them worked.

Delete all items in Options/Security/Passwords/Saved Passwords, check that cookies are enabled in TB Options/Privacy, use imap.mail.yahoo.com on port 993 or pop.mail.yahoo.com on port 995 for the incoming, smtp.mail.yahoo.com on port 465 for the outgoing; SSL/TLS security, OAuth2 authentication and User Name = email address for incoming and outgoing. Then restart TB and enter the account password in the OAuth browser window when prompted.

I started with a brand new empty reinstalled Thunderbird with nothing to delete. I used all the prescribed configs but do not know what OAuth2 is or where to put it.

Go offline from File/Offline/Work Offline, then File/New/Existing Mail Account, enter your credentials, click Manual config. On the next screen, enter the servers, ports, security and User Name. Click Advanced config., select Server Settings for the account, set the authentication method to OAuth2, and also for the Outgoing Server (SMTP). Close Account Settings and go back online from File/Offline/Work Offline. Enter your password when prompted when TB connects to the mail server.

I tried to follow the instructions. TB does not accept either .sbcglobal.net nor .mail.yahoo.com as the server. And there is no option for OAuth2 on the outgoing.

You have to manually enter imap.mail.yahoo.com or pop.mail.yahoo.com for the incoming server. If you follow the directions and open Advanced config., the authentication for the outgoing server can be set to OAuth2.

I finally fixed it myself. The correct incoming POP3 server is pop.att.yahoo.com and the outgoing is smtp.att.yahoo.com. OAuth2 is not accepted. Instead of usual password I had to go to MyAt&T to get a secure key to enter instead.