What do you mean?
Cash credit?
That only works for certain services like MMS, international calls etc.
If you want membership you need to buy that with your cash credit.
Or did you buy a €20 membership already?
If so then the new membership will not start immediately if you still have a current membership in place that has not yet expired.
What does your online account say about your current membership and expiry date?
Hope this helps.
