Free Online STL Repair
STL files may have gaps (/holes) or loose parts such that they cannot be 3D printed when the mesh is resolved. Modifications are required to deal with those gaps and issues. Some software includes option to automate this, but some don’t.
If you are not using anything sensitive, then uploading to Formware’s free online tool to auto repair it is an option that seems pretty reliable.
See also Formlabs’ STL repair guide