Well, I found the reason after some trial & error. If line 16 on Schedule D is $0 or a loss, the program skips lines 17 through 20 or 21 and zeros them out. I had a loss on line 16 as it were, and when I tried to enter a value in line 19, it gave me the error. I know now that Section 1250 gains are offset by long term capital losses 😉 Learning the hard way I guess...
