FWIW, I'm able to get the routing the show up on Google Flights and it looks like it's bookable via the Delta website (though I didn't try to finish the transaction so I guess the error could still occur).
Although the outbound books into V, and the return books into M (with a price of $595 for main cabin)
Then my guess is that on the return, K class (or H or Q) isn't available due to married segment inventory, although there could also be problems (unlikely) with minimum advanced purchase requirements for travel in mid-November.