You don't give any detail so it's hard to know what the problem is. However, one possibility is that you are using gmail and google expects a form of multi-factor authentication. There was a long thread in the emacs help mailing list about this and how to get it working from gnus.
-- Eric S Fraga via gnus (Emacs 29.0.50 2022-07-20) on Debian 11.3
