that the CAD model is not sufficiently prepared for meshing/simulation could be the reason, but it doesn’t have to. Did you already check why the meshing fails? There are two main places where you can check on the mesher’s progress. The first one should always be the Mesh Operation Event Log that summarizes the execution of the meshing operation and highlights potential problems.
If the event log does not give you the answer yet, you can proceed and check the Detailed Meshing Log where the detailed (sometimes too detailed) output of the mesh operation is visible:
Typically, the hex-dominant meshing for flow simulation is very robust and should also handle unprepared CAD models during meshing. It can happen, that the mesh is not valid (e.g. the wrong part of a flow domain was meshed if the model is not watertight) but the mesher rarely really fails. The tetrahedral meshing for FEA simulations is stricter in terms of the CAD quality it expects. There are some hints summarized in the documentation here:
If you an share your CAD model here or at least images of it, there are many SimScalers that will give you hints on what you could do to improve the CAD model for meshing. Would that be possible?