I use 123D design for my designs. I’m facing a problem when i import an STL file in 123D design and convert the mesh to a Solid. When i do that, it says The number of facets are more and it might reduce the performance of the computer and when clicked ok the cursor keeps spinning for a while and then says couldn’t convert.
In general, STL files are generally not meant to be edited any further and it’s difficult to treat them as anything but a mesh. Try importing the STEP file instead.