How to use a Apple credit memo?

Hi, I received a credit memo for an Apple Watch I brought in June as there was a courier error so was sent a replacement and they sent a credit memo as a gesture of goodwill. I paid by card but the payment never went back on the card and on the invoice it said the credit would go as a cheque refund which I never received. I tried using the memo number as a gift card on the website when I tried to order something and it still wouldn’t work. So I just wanted help as to how I could use this please?

thankyou