"You entered a refundable Flight Credit. You can request a refund to the original form of payment." I can't put it towards... a flight?

Canceled an itinerary last fall and got issued a flight credit.

Figured I'd use it towards a trip I'm about to book, and upon trying to do so, it give me the error:

You entered a refundable Flight Credit. You can request a refund to the original form of payment.

Presumably this means I have to go through the prefunds site? Seems odd that I can't just put it towards a cash fare I'm trying to buy.