You need to contact support and inform the buyer that you have delivered the order as requested and therefore they are not entitled to a refund in accordance with the TOS. Hopefully fiverr support will be helpful and will force complete the order on your behalf.
If support choses not to help you, you basically either have the the option to push back against the buyer and attempt to get them to accept the order, or you can cut your losses and give them the refund and take the cancellation. The one risk to trying to get them to accept the order, is it’s always possibly they will leave a bad review. So you have to assess the situation and the buyer and figure what you might be able to get them to do and what their reactions might be.