I just had this on Christmas Eve flight. I booked a cheap award weeks ago for the last flight of the night ATL-PIT. I called on the 23rd and asked to confirm to the first flight of the day which was Y4. Diamond agent said I couldn’t because NL was all that was available. I HUACA once and got the same response. Agent was nice enough to walk me through it for the future. If you do a dummy booking with miles and see NL, best you can do is SDS. In my case, I did the SDS, and got it.