I was running into this same issue and I think I found the problem. When filling out the basis worksheet, I had an ordinary business gain that was listed in part I but then a cash contribution from box 12 A in my K1. This went into part III. I kept getting the "cannot have a gain in Part I and a loss in Part III" error when I would do my federal review
I went back into the basis worksheet and the ordinary business income line in part III has a "0." in the loss section, I deleted the "0." in that field and then left it blank. This allowed the form to validate.
I think the issue is that the validation is saying you cannot have anything in that field for Part III if you have the field filled out in Part I even if its zero loss
