You’ve gone and applied a credit memo against an open AP invoice. A couple of days later, you realize that you made a mistake in applying that credit. So to fix this, you would go back to the credit memo voucher and select the “Apply Credit Memo” button (or edit menu) and undo what you previously did…BUT the Apply Credit Memo is disabled (greyed out)!
Tip: Check the status of the credit memo voucher. It should be set to “Pay”. If it’s not, the apply credit memo option will be disabled