How do I resolve 'Authentification failed' error?

For any non-icloud email account with such a message, go to Settings, Apps, Mail, Mail accounts, find the bad one, remove the account.

Add the account back, using where possible only the email address and the correct password. Don't mess around with server settings.

Contact Yahoo. That's not an Apple service.

Generate an app-specific password, remove the account, re-add it, and use the app-specific password.

Yahoo discusses it here: https://in.help.yahoo.com/kb/new-yahoo-mail/enter-yahoo-generated-password-sln15241.html

If problems persist, contact Yahoo Support.