You get this message while you are using the account wizard. This message appears when the mailadres you used is not in the database of Thunderbird. When i use a fake @mailfence.com adres Thunderbird find the configuration so maybe you made a type error. Under the yellow line with the error message should appear a manual configuration screen. You can add there the credentials of your mailprovider.

Hope this helps

Problem still remains....... Unable to add my Mailfence email account to Thunderbird deb install on Ubuntu 24.04.3 Set Up Your Existing Email Address in Thunderbird returns " Thunderbird failed to find the settings for your email account" yet my web mailfence is accessible

When Thunderbird cannot find settings, the next thing to do is enter the settings yourself. Have you tried that?