Have you tried setting up your Yahoo account via the “Automatic email wizard” ? As should now work for Yahoo accounts with the recent latest versions of eM Client V8 and V9.
See also this thread for setting up Yahoo Accounts.
https://forum.emclient.com/t/yahoo-security-for-third-party-email-clients/66087
You may also need to "generate an “app password” and “not your normal password” too when using the automatic wizard or manual other option as some users have now found due to Yahoo changes recently.
(Generate Yahoo App Password"
https://help.yahoo.com/kb/generate-third-party-passwords-sln15241.html