I need some help. What is a non-manifold body? I just spent an hour drawing this design and it won’t let me extrude it.
I might be wrong, but it seems you have selected some edges at the top right part of the sketch which I think would prevent extrusion.
Google to the rescue! I found an old conversation in this forum on the same topic and realized that I had a triangle touching another shape at the corners. I deleted that and moved it over a little bit and it extruded perfectly. I feel so accomplished 
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Well done 
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