What should I do if my Roblox Premium purchase fails?

So I tried to buy Roblox Premium (11,99€) today and I was met with an error telling me the purchase didn't go through. I thought "Okay let me try again" and the app told me that I didn't have enough funds (in Apple fund). I checked and saw that I only had 3,01€ in my funds although I just added 15€ to my account. I assume that the money had been withdrawn even though the purchase did not go through. I can't request a refund in Apple Support because it considers the purchase as "not happened" and all I can do for now is contact Roblox Support if I could get Premium or not. What do I do now?