When were you last able to successfully buy a €12.99 plan?
Are you sure that your payment card is still valid, there's cash in your account to cover the cost if it's a debit card, the card isn't frozen via your online banking, etc.?
What happens when you try now?
Any error messages?
What happens if you use this link?
If you need to contact 48 support about this then see the links in my signature below.
