Cannot buy robux with Apple gift card; Cannot buy robux with Apple ID balance

I cannot buy robux with my Apple ID balance. I bought a Apple ID gift card from seven eleven and I redeemed it on my iPad Air 3 but when I try to buy robux it will say:

Your purchase could not be completed

For assistance, contact iTunes support at www.Apple.com/support/iTunes/ww/.

I don’t know what to do, this was a gift for my relative but now he can’t get it. If I can’t buy anything, I hope I can get a refund or something. Please find a way to help me, thank you!