In Fusion lofting is a quick, easy and intuitive operation. For example, I’m trying to loft the top of a guitar. I have a horisontal plane, and a curve at 90 degrees (like in the picture). In Fusion it is simply a matter of selecting the three planes
I followed the tutorial and get exactly the same result in Shapr3D. The only difference that Shapr3D doesn’t support surfaces, so it doesn’t have such thing as surface loft. But that’s not a big problem in this case because you need a body as a result, not a surface.
But your profile on the first picture totally different and it won’t work in F360 as well. (To be precise, it will technically work, but it will give a completely incorrect result. Shapr3D simply refuses to do it right away.)
Alex already mentioned it above, the shape of your guitar is more complex so it will not loft following the Fusion tutorial.
Tutorial is a traditional guitar, its basic oval shape without pointier ends, different approach would be needed. I would look for electric guitar tutorials. You can also search the forum and see what others have done.