There are routes where DL does not sell revenue tickets, but still offers award redemptions. The current website award search has various issues right now as has been documented in several other threads. It always defaults to a flexible dates award search (even when specifying exact dates) and there are a number of routes where flexible dates search always fails, but where an exact dates search will succeed. There are a couple of workarounds to get the website into an exact dates search mode for awards -- the simplest is to just use m.delta.com. The other workaround is to use the app. However, after doing a number of exact dates searches on this particular route, I have yet to find one that succeeds. So I'm inclined to believe that DL does not consider it a valid award route.
I tried the exact date search first, and got the same error message, before I tried the monthly calendar search, just in case it was a "won't work on a specific date" issue.