It seems like you are having your main frame as a shell rather than solid. Therefore, only the solid parts are meshed. Can you try to re-upload geometry as a single solid i.e. merge them together if you have to assign the same material to your geometry. You can have a look at this forum post for more info.: Merging solids in Onshape and meshing
You can also have a look at this project for setting up your case: https://www.simscale.com/projects/ahmedhussain18/trial_2/
Hope this helps. If you have any question/s, feel free to ask.