I am an admin on a fairly new site (approx. 6 months). All of a sudden, gmail won't let me send emails from my workspace account. I have a google domain. When I log into the admin console, I get this:
I don't know why the Admin Console instructions say that.
Go to Google Domains and in Custom Records, create a new DNS TXT record by copy/paste from the DKIM page (Admin Console: Gmail / Authenticate email page). Once the new TXT record is created, then go back to the Admin Console to press the Start Authentication button -- it will verify the new TXT record by querying DNS for your domain.