You can only create a credit note, if you already have created an invoice.
Follow the next steps to create a credit note:
1/ A) Go tot administration part of the booking details and click on the 'x' : credit note
(Not possible for hotels in France)
B1) Go to Administration > invoices > search for the invoice
B2) Edit the invoice
B3) Click the button 'create credit note' (you see on the right)
2/ All items on the invoice will be credited automatically. Remove the items you don't need on the credit note.