Letting your customers cancel and transfer their own tickets in self-serve is a great way to save customer service time, and to offer your customers flexibility. Sometimes your customers may get stuck, even if you've enabled cancellations in your self-serve settings.
If your customer can see the error 'payment pending, contact event organiser', it's probably because they paid for their ticket with an offline payment method.
If this is the case, you can enable self-serve for your customer by marking the pending payment as complete, or you can help them manually from your Ticket Tailor 'Orders' page.