I also got this message (see photo). So I called the PayPal customer support phone number and they were super helpful and added it with no problem! Turns out I had used that credit card with a different PayPal account associated with a different e-mail. I had closed that PayPal account years ago, but because the card number had been in their system at one time, it could not be linked to my current PayPal account. The PayPal representative had to reset the card on their end. The rep also told me you can only have 4 debit and/or credit cards on your PayPal account. However, if you have a verified bank account in your PayPal Wallet, then you can have up to 8 debit or credit cards on your PayPal account. This is as of March 2018.
... View more