I will presume you are familiar with the F1 in Schools tutorial "Flow Over a CO2 Dragster" and you are using a STEP (.stp) file to transfer your geometry from your CAD system to Caedium.

I would guess there is either a problem with your geometry, face meshes, or volume mesh.

In terms of geometry you need to ensure you have a single, watertight (i.e., all edges are connected to 2 faces) volume of the air around your car as in the tutorial.

First check your volume is watertight:

1. Select all edges
2. In the Results Tool Palette select the Connected Topology
3. In the Properties Panel ensure Connected = 1 (i.e., edges connected to only one face) and click the Add Connected Top... button

If any edges are still selected then you don't have a watertight volume. You will need to connect them either in Caedium or your CAD system.

If all is well then next you need to review your face meshes and your volume mesh following the procedure outlined in the section "Specify Meshing Parameters" in the tutorial "Flow Over a Double-Sided Membrane". Note you want to correct face mesh problems before moving on to the volume mesh.

If you find problems then review the FAQ "How do I fix mesh and solver failures?". Given you have a geometry or mesh problem ignore the solver specific suggestions.