The first place you need to look, is in the Synthetic Records area of your domain. It's the area right above Custom Resource Records and you'll find a
> G Suite all by itself towards the bottom. Click to expand this out and look for the record you're trying to add, because it might be in the Synthetic Record. For example, if you're trying to add MX records for G Suite, or Authenticate the DKIM key for the G Suite email. These might already be in the Synthetic Records and enabled. This is one reason you might get this error.
If you hit EDIT to the right, of the Synthetic Records, you can either enable or disable Email or Subdomains. Maybe you're trying to add the "mail" CNAME and it's already enabled in the Synthetic Records "Subdomains" area.
Point is, if you're adding any Google related records or sub-domains that "could" be used for a Google app (Mail, Calendar, Docs, Drive etc) then this is the first place to look.