Your query is not clear.
You bought a €5 voucher.
Were you able to topup your cash credit balance with that €5 voucher?
If not how did you go about this and what happened/went wrong?
If you were able to use the €5 voucher to topup your cash credit balance are you having some other problem when trying to buy membership?
If so what?
And how are you trying to buy membership?
If you're trying to do this by text but your last membership has expired then that will not work.
But in that case you can log into your online account to buy membership and pay for it from your credit balance.
What does your online account say about your membership status, expiry date and allowance balances including cash credit balance?
Hope this helps.
