I see in the part list of your geometry that you have some surfaces. CFD simulations calculate fluid parameters for elements in a 3D mesh and so all your parts must have thicknesses. Can you explain what the aim of your simulation is? You have chosen the incompressible solver so it looks to me like you want to simulate the aerodynamics around the car but if this is the case, you need to mesh the air around the car (geometries --> add geometry operation --> enclosure), not the car itself. Furthermore, your CAD model has interference (where two parts intersect) which is an absolute no for CFD and if this is an aerodynamics simulation there is no need to model the internal structure of the car, you would be better off importing the car as a completely solid body.
Edit: I see you say this is just for fun and you have no engineering experience, well start on a simpler geometry, in fact start by going through the tutorials on the website. Although simscale is easy to use as far as CFD packages go, it requires knowledge of proper CAD preparation and the fundamentals of fluid dynamics. I think it would be a lot of work for a ‘lay person’ to become proficient because of the background knowledge required.