After accumulating DL miles for a while, I am trying to redeem some for the first time.

I'm considering a domestic trip with a stopover A-B-C-A to maximize the value of the award. I was very pleased to see that delta.com has a page to do this online, especially with the new phone booking fees. (AA.com only lets you book simple roundtrips).

Anyway, I tried DFW-SLC-MSY-DFW on January 14/20/23, and I get an error:
There was a problem booking your itinerary online. Try selecting a different class of service or direct flights. For assistance, call 800-323-2323.

Is there a problem with this itinerary? Or am I doing something wrong? If I substitute EWR for MSY, it searches fine, so it looks liek even out-of-the-way stopovers are ok. And if I search for a paid ticet on that same DFW-SLC-MSY-DFW route, same dates, there are options including nonstop DL or DL Connection flights on all legs; so it looks like the routing exists.

So what's the problem? Any ideas?