One piece of advice: I had the same issue a few weeks ago, when I wasn't able to 'cancel' an upcoming flight online and instead was prompted to fill out the refund request form. I filled out the form and received a response from Delta support a few weeks later saying the e-credit had been issued. When I checked the e-credit online, however, the value was $0. After calling Delta support (only a 45 minute wait on the Silver medallion line...), they said that I had been marked as a no-show even though I had submitted this refund request 3 days before the flight. They managed to escalate to a manager and gave me a one-time exception and reinstated the full value on the credit. She said I had to call in if this happened ever again.

Anyway, bottom line is: if you get an error when canceling online, CALL. The wait times are atrocious, but at least you'll get it taken care of.