Family memberships are not available for G Suite accounts. Sign in with a personal Google account or buy an individual membership to continue.

All I wanted years ago was an email address with my vanity domain. Had I only known I was shunting my whole family into a Bizarro Elgoog world...

Google really screwed everyone that used this; it was marketed towards families/small groups, and now everyone is forced into Google Workspace with massive breakage for family-like stuff. Thanks Google.