If I were to guess, OP was a part of a different plan before and is switching to a new one. I think family plans are cheaper overall if you max out the number of people and everyone contributes, so families or groups of friends all link their accounts and pay fair shares? (EG: Nintendo online is $20/yr, but $40/yr for a family plan up to 8 accounts. If you get 8 friends together, that's $5 per person, significantly cheaper than $20.)

So if I'm understanding the plan and reading the error correctly, I believe OP was on one family plan and wanted to change their family to a different set of accounts, and Google said no. Which is indeed shitty, but it's likely buried in their fine print somewhere (which doesn't make it any less shitty).

Or maybe OP wasn't even a part of a family plan to begin with. OP, feel free to correct me.