What plan were you on previously?
When did you last buy one?
If you're on the €3 discount on the €12.99 plan (= €9.99) for 6 or 24 months and didn't keep membership plan auto-renewal enabled then you lose access to the discount in case that's relevant here.
Cash credit on your account can be used to pay for stuff as long as the cash credit balance on your account is greater than or equal to the cost of what you're buying.
E.g. if you have €13 cash credit on your account and are buying a €12.99 plan then you should get the option to pay using your cash credit at checkout time.
If you have less than €12.99 cash credit then you won't get that option.
If you need to contact 48 support about this then see the links in my signature below.
