I can’t get robux

You will need a valid payment method.

Add a payment method to your Apple ID - Apple Support

Remove a payment method from your Apple ID - Apple Support

You also will not be able to download apps or even update them if you owe money. That will need to be paid before you will be able to use the App Store or make any additional purchases.

If these "Robux" are in-app purchases for Roblox, then you're going to be charged real money to get them.

https://en.help.roblox.com/hc/en-us/articles/203313200-Ways-to-Get-Robux

I don't see how you expect to make in-app purchases without a valid payment method on file. From what I've read, you often need a valid payment method on file just to download "free" apps!

Why won't Apple let you?