The user e-mail the error refers to is the mail address of your admin user. In order to send a test mail, that mail needs a recipient, which is taken from the admin user mail field in the profile.
I’m not sure what you mean by “NOT interested in using email with NextCloud”. The error does not refer to setting up the Mail app and all that, but just to provide the contact e-mail the “Welcome mail” should be sent to.
Have you entered an email address in your personal account settings?
As long as you’ve enabled an app access to access your email account, it should be fine. You should find further information about how to do it, by using the search function of this forum.