Perhaps, this tutorial will help identify and resolve the issue: Aerodynamic Flow Behavior Around a Vehicle Tutorial | SimScale
Note that the simulation domain for a fluid dynamics simulation is the region occupied by the fluid (gas or liquid). Perhaps, the step missing is to delete the solid parts to be able to use the model in an analysis type that requires a single fluid region. That is, after the fluid domain is generated, the next step would be to delete the solid parts from the geometry. (See section 1.2 of tutorial and/or see this documentation on performing the ‘Delete’ operation: CAD Edit | CAD Preparation & Upload | SimScale)