You have to have enough cash credit on your account to pay for the membership plan in one go.
E.g. you can't do a split payment where you used some cash credit and pay the balance with your card.
E.g. if you're buying a €12.99 membership plan but only have €6 cash credit on your account then you'll need to add another €7 cash credit in order to be able to get the option to pay using cash credit.
Does that address your question?
If not then please clarify what exactly is the issue.
If you need to contact 48 support about this then see the links in my signature below.
