It might be version issue. What stl version you are using?
did you try those:
https://optics.ansys.com/hc/en-us/articles/360034901953-STL-import-Simulation-object
https://optics.ansys.com/hc/en-us/articles/360034924733
If problem still exists, please send us your stl file together with your solidwork version and software version to the support email.
Later please write your post here https://forum.ansys.com/forums/forum/discuss-simulation/photonics/ for photonics software (Lumerical).
You are navigating away from the AIS Discovery experience