A user in my discourse requested to have me change their username. When I went to do it, this popped up on my screen.
AI-generated summary
KirbyKing encountered an error message, “Username needs to be updated on authentication provider side,” while attempting to change a user’s username in Discourse. Lilly explains that this indicates the forum has the auth_overrides_username site setting enabled. This setting implies that an external authentication provider manages user logins, meaning Discourse relies on usernames from that external service. Consequently, username changes must be made on the authentication provider’s side, not within Discourse. Lilly suggests checking the Login tab under admin->all-site-settings to verify the configuration.
it means exactly what it says - that your forum has the admin-site-setting auth_overrides_username setting enabled, which means that another authentication provider service is authenticating your users and Discourse is using their login usernames from that site. their usernames need to be changed on the authentication provider.
maybe check the fields in your Login tab under admin->all-site-settings