If the amount of cash credit on your account is greater than or equal to the cost of whatever you're trying to buy then you should get the option to pay for it using your cash credit rather than you payment card at checkout.
If the amount of cash credit on your account is less that the cost of whatever you're trying to buy then you won't get that option. I.e. you won't get the option to part pay for something using cash credit and the rest on your payment card.
If you're using the app then what happens if you try the website?
Or vice versa?
How much cash credit do you have on your account?
Is you can't get this working then you'll have to contact 48 support for further assistance.
See the links in my signature below.
