It may be the case that having multiple entities reporting QBI, or having multiple of boxes 1/2/3 filled in, are NOT THE ONLY WAYS to generate the error that is the subject of this thread. In my case, there was only one entity, and only box 1 was filled in. I had 2 items in box 20 labeled with "Z", but I ended up putting both of those on different but corresponding lines in the Section D1 table I described in my previous post.
I think there is a bug in TurboTax, where it failed to propagate the information previously entered into boxes 1, 2, 3 etc. into the Section D1 table in the full schedule K-1 form. Once I added some info there, and then went back through the interview, a new screen entitled "We need some information about your 199A income" popped up showing that table and allowing me to enter values there. That had not popped up previously. The values I entered on two lines on that screen were the two values marked as "Z" in box 20. Seems like this table should pop up whenever anyone has an entry marked Z in box 20. For now, as I suggested, you can just open the K-1 in Forms and edit section D1 directly.
