I created a new simulation project called 'CISTERNA 3D':
CISTERNA 3D-PLASTIC VESSEL LOADING BY EXTERNAL HYDROSTATIC FORCE
More of my public projects can be found here.
Can you try to merge all solids of your geometry in to one and than try to mesh it. For performing that you can have a look at [this] (Merging solids in Onshape and meshing) forum post.
If you have any question/s, feel free to ask. Happy Simulating!