I am running into the exact same problem, running 3.98 on DNN 5.6.0
Any resolution? I tried with a header row in the xls and without (the attached is without). I am thinking there is a required field that I do not have, but not sure...
To note, the category exists, but the subcategories do not (trying to get it to auto-create).
I did upload data successfully previously to this site (earlier version of SmithCart).
Attaching xls.