If you can’t select a credit card when you’re checking out with PayPal, it’s because the amount in your PayPal balance is greater than the amount of the purchase price. PayPal only allows you to select an option to use the existing account balance or to make an instant transfer from a bank account. This is annoying.
To remedy this, you need to reduce the amount of money in your PayPal account. You can do this by transferring funds into your bank account.