Read How to Repair an STL File for 3D Printing, check non-manifold geometry, or check for open edges.
Safe STL cleanup
STL Repair Tool
Remove duplicate and degenerate triangles, and correct simple orientation issues before checking your STL again.
Upload an STL to clean safely
InspectMesh safely cleans only certain STL issues. It cannot reliably fill holes or repair every non-manifold model automatically.
Select a non-empty .stl file.