What's the issue? Says "The family payment can not be used for this purchase"
I don't want my credit card on their devices... Why is this not working?
I contacted support, i bounced around 3 teams, and they say remove payment and try again..
I removed all payments, put back, same issue...
what exactly did you try to buy with your family payment method? You can find some restrictions in this article. Under "troubleshoot problems" you find also a part to try to change the payment method on Google Play. You can also try out if that works for you (you probably tried it directly via pay.google.com)?
Otherwise, as you have already a "ticket" open with Stadia Support, I recommend you to push back on that ticket if it still doesn't work, usually they should have the possibility to investigate this further and provide you with more troubleshooting steps you can try.
What you can also do is browsing the Google helpcenter as maybe others had the same problem like you already and you might find some additional helpful articles or forumposts there as well.
Wish you the best!