There are three options I see here:

1.You could try to login to "MySwtor", then click on "MyAccount" and then "Subscription", then "show payment history" and add through that page a new card and delete the former one listing. After that; go back to regular purchase method (it's quite an old "trick", hasn't worked for all and might not be working anymore).

2. Call costumer support.

3. Buy a gamecard over Origin although they cost quite some money, so that should rather be your last resort.

I don't know whether or not that Error can have other reasons but here is my guess of what to do.