I can't make any purchases with my card.

If you are having trouble registering your card, or the purchase is not being completed even with the card registered, see if there is a warning at checkout.
Check for possible problems with the card and make sure the data entered is correct.
You can check with the bank that issued the card to see if there are any pending issues or if it is blocked for online purchases.

If the card has already been used before without any problems, the advice is to contact the purchase site to check that there is no security validation to be done.