I'm relatively new to solid works and can't seem to get this part to thicken.

The part was originally 2 solid pieces,  then I deleted the faces and lofted together. I have knit the remaining pieces and then mirrored. The error I'm receiving is " failed to offset or face couldn't be detected" yet the preview shows exactly what I want.

( The part is for a 3D print)

Any ideas?

Cheers,