Gmail OAuth client ID and secret are required (Admin → Settings → Email)
Site mail configuration check failed in Gmail mode because OAuth client ID or client secret is empty. Message includes the Admin → Settings → Email path.
Why this happens
assertSiteMailConfigured requires xoauth2ClientId and xoauth2ClientSecret before consent can work. A shorter variant without the Admin path is thrown when building the transport at send time (covered in a later phase).
Diagnose and fix
1. Admin → Settings → Email — set mode to Gmail.
2. Paste Google Cloud OAuth client ID and client secret; save.
3. Set the Google account email, then complete Connect Google (next errors if refresh token missing).
4. Follow Gmail XOAUTH2 for redirect URI and API console setup.