User is trying to create a negative expense entry in Core and link it to a credit card account for a refund/credit transaction, but gets an error message.
Core does not allow you to link negative expense entries to credit cards from the Expense Entries screen. If you want to enter a refund transaction, the proper way to do that is from the Credit Cards screen. You should create a credit card transaction of type Refund/Credit to handle the refunds. Check the Core Help for details. Then Core automatically creates a negative expense entry for such transactions.