A user in my discourse requested to have me change their username. When I went to do it, this popped up on my screen.
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
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