In app purchases with apple gift card

See this, How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support Some purchases cannot be made wit the gift card, and there is a section in this support document that speaks about that and has other links.

You need to add a credit card. Not all apps allow for in-app purchases with an iTunes Gift card, and there could also be a verification issue if you do not have a credit card added to your Apple ID. Apple uses that to verify that you are using the correct iTunes and App Stores.

I redeemed my apple gift card but it wont let me use it on roblox is says u need to add a credit card what do I do?