Cannot buy VBucks with ITunes credit

My son has loaded up $40 credit onto his iTunes account on his IPod -when we check setting, payment, iTunes credit shows $40 available.

He has played Fortnite for a long time and up until now he has purchased VBucks(gaming credit) with my credit card. I have removed my credit card so he can now use ITunes money.

Anyway, when he goes to purchase the VBucks it says he has no credit and takes him back to iTunes Store to purchase more when he already has an active balance?