Welcome to the community forum and thanks for posting.
I am sorry to know that you're unable to add a card. Kindly check this help center article to know why there could be an issue and to know how this could be resolved. Here's the link to add a card to your PayPal account. Please note that you cannot add virtual bank\cards as they aren't compatible. If the issue still persists, please reach out to our customer support team through the available contact options on our website.