So this box I am modeling has a lip on it which needs to curve and connect at the corner. I have tried connecting them with a loft and the preview result looks pretty good, but it won't create because it says it would cause zero thickness geometry. Looking at the model and the preview I can't really see where the problem is, so I don't really know how to go about fixing it. Any ideas on where to start, or alternative methods to make this work? 
If you set it to not Merge - will it create the body?
My preference would be to add Fillets as features at the end.
You might also try lofting individual surfaces and then Knit.
Can you attach the *.sldprt file here?