In short, the volumes should be watertight & solid for simulations. As I can tell, the underfloor of your car consists of a simple sheet, so that’s causing issues. I’d recommend going back to your cad model and adding some thickness to the part.
The second problem is related to the enclosure operation. For Incompressible analysis, we need a single volume to run simulations (the flow region). The other solid parts should be removed from the geometry, otherwise they will cause a multi-region mesh error, which is the error that you ran into.
To solve this, please toggle off this option in the enclosure operation: