Can you please try to merge your solids in to one? I have seen that you have done already in one of your other project, but the file you uploaded was an STL which can’t be meshed via tetrahedral mesh. Please try to upload the geometry in .step or .stp format and make sure it contain solid, not only shell.
For merging the solids in to one, you can see [this] (Merging solids in Onshape and meshing) forum post. If you have any question/s, feel free to ask. Happy Simulating!