I could not pay the game by card. I am trying to buy them part and it say there is a problem. Do you have any idea why?

You will need to be clearer, what type of card, what is not happening? Note, you cannot buy DLC or addons without owning the base game, this is what usually stops it. Also, maybe try another method, PayPal for instance. If not, contact support.

This is not customer support, it is a forum for customers. You need to talk to Gog customer service:
https://support.gog.com/hc/en-us/articles/212805865-I-tried-to-make-a-payment-and-it-didn-t-work-What-can-I-do-now-