You can only create a credit note, if you already created an invoice.
Follow the next steps to create a credit note:
1/ Go to the administration part of the booking details and click on the 'x' : create credit note
2/ Click 'ok'
3/ All items on the invoice will be credited automatically. Remove the items you don't need on the credit note with the icon and keep the items you want to add to the credit note:
4/ Click on save
5/ Click on generate invoice to create the credit note PDF
6/ The credit note will be generated and saved in the administration part of the booking