Some of the expense entries don't allow the user to tie up with a credit card.
This happens when you are trying to link a credit card to a system vendor bill, not a regular expense entry. You cannot tie a credit card to an expense after creating a vendor bill out of it. If you want to pay the expenses with your credit card, you can directly do so from the Bill Payments screen, or in case of reimbursable expenses, you can do it from the Checks screen. Check CORE Help Center for details.