I don't think you need to go thru the sew faces step, SC does that for you when you import the iges file.
I remember when I first started using SC I would get the same error "Failed to Sew Faces Into Solid Given The Specified Tolerances." but for the life of me I can't remember what I was doing to cause that and how I fixed it. Seems like it had to do with the operation I was using and what geometry I was picking in job assignment... I think I found that under that type of operation I was choosing the wrong geometry, either face or edge? I just tried to quickly reproduce it SC but could not. Can you post the SC project file or tell us what operation you are using and what geometry you are picking for the operation under job assignment?
Sorry can't be of more help.