Thanks for the link.
The error is caused by the parts representing the kart being present in the model. For the aerodynamic simulation, the model can only contain the part representing the flow region.
After creating the flow region, you should delete all the parts representing the solids, with the Body - Delete operation in CAD mode.
On the other hand, I noticed that your model contains some surface parts that could not be automatically converted into solids during the import process. This non-solid parts will create problems in the creation of the flow region and in the simulation, thus you need to fix them or convert them into solids before import.
Also, I suggest you to simplify the model by removing small details that are not relevant for the global aerodynamics.
Please use this tutorial as a guide on a good level of detail and aerodynamic simulation: