Your Purchase could not be completed sign/Child Account

So, my son is part of the family sharing. When he wants to buy something on Brawl Stars, we get a Purchase could not be completed message.

1. "Ask to buy" is successfully set up on his phone, and it works. I approve of his purchase, he gets a confirmation and then "Purchase could not be completed message".
2. The payment methos attached to MY account is valid and updated.