convert imported *.stl into solid

I am importing a rather complex geometry into COMSOL from Solid Works. After some tinkering with the import controls, I managed to import the object but I do not think COMSOL recognizes it as a solid. When using the physics, I can only click on edges, not faces.

What do I need to do to 'knit to solid' or make the object into a solid as recognized by COMSOL. Any direction would help a lot.

Should I maybe open it in a different software and convert it into a solid there?