Trying to combine two costumes without "Mesh Import Error"

I'm new to Sims 4 mod creation so I followed this YouTube tutorial to try and figure out how to combine two outfits together. I downloaded a mod that unlocks the skeleton from the jungle pack but as a costume and I'm attempting to combine that with the butler outfit so I can make a skeleton butler costume. I thought I did everything right in blender but when I try to export the mesh back into S4S it gives me an error saying "One or more of your meshes requires a UV map but does not have one." Not sure what I am missing, any help at all would be very appreciated.