In general CAD formats such as STEP or IGES as significantly more robust and recommended over faceted and approximate formats such as STL. That said:
Grid generation error.
Could not find mesh to import.
is a general error message indicating that mesh generation has failed and a mesh could therefore not be found to be imported.
Without any actual information about your specific model and system one can only speculate, but it is quite likely that there are issues with your STL geometry model such as gaps, non-watertight domains, or duplicated and overlapping facets.
You can possibly try to find some geometry/mesh repair tools to pre-process your STL model before meshing, or try to manually mesh the STL file (in for example Gmsh) and import the finished mesh instead of the geometry.