Using the TET mesher, I am pretty sure that your geometry file still needs to include the domain walls (Background Mesh Box)…
To do this you need to Boolean subtract your car from a BMB sized watertight domain solid (if your CAD software allows this, however, you might need to support your car on a very small string that allows the Boolean subtraction yet the small string will never be meshed in SimScale because it is smaller that your finest refinement level)…
I am pretty sure you will not find the above info at that link… 