FWIW I had the same problem a few months ago, and they did eventually sort me out. This surprised me, given Google's reputation. I was on the 1-month "free trial" membership, and always planned to switch to the single-person membership if they didn't get it sorted out, so I wasn't really out any money during that time.
(And in fact, what actually happened was they hadn't sorted me out by the time the free trial was up, so I cancelled it and switched to the individual membership. They managed to get things sorted out a week or two after that, but I'm still on the individual membership; my wife just shares my account ID.)
Once I subscribed to YouTube Music (paid!) family plan, but my wife's account would always say that she is in another country and can not join the plan. I tried everything - the support never even bothered to reply to my emails. I cancelled the plan and since then I keep seeing the same ads for the premium service every time I open the YT mobile app.