I’ve had that happen before. I think it’s a glitch. If you hit “discount” and then hit cancel and exit out of that screen it will let you void the transaction after for some reason. I’ve had it happen with gift cards and a bunch of times during happy hour when a partner scans someone’s sbux card that doesn’t have funds first.... order gets stuck and doesn’t let you void or refund.

I've seen similar errors before, where it won't let you void or cancel the transaction without scanning the cards again. Last time that happened I just did a refund for the exact amount as the misc SKU and made a note in the DRB to explain what happened.