I think I understand now. I normally have a gain on my K-1 from Section 1256 contracts. This year I have a loss. I can carry it back and spread it over previous years by amending my tax returns. By unchecking Box D, I gave up the option of going back and getting a refund by spreading the loss to earlier years, and the error goes away.
I think I understand your statement about combining numbers. When I input my data, I can enter a -87 on line 11zz. Or I can combine 11zz and 13zz and enter -87 for line 11zz. Is there a reason to combine them?
Thanks for your help. I get no errors now.

the instructions you presented say the 11 and 13ZZ should be combined and reported on schedule E line 28. so you can enter 11 and 13 separately and the link each to the form or do a net entry on 11 which would require only 1 linking. in either case the net result should be the same.
the error on the 6781 may be due to the fact that if c/b is elected, the amount of the carryback must be entered on line 6 of the form as a positive number. it can't be left blank but can be any number up to the current year 1256 loss.