Hello there, @Nando1.
Currently, the Credit Note option is unavailable in QuickBooks Self-Employed.
In the meantime, you'll have to leave the status of your invoices as unpaid until they get back for your driving services.
You can check out this article for more information about QuickBooks Self-employed:
Please don't hesitate to reach back out to me if you have any other questions and I'll be around to help you out.