Hello and welcome to SimScale!
I took a look at your model geometry. It looks like you have a plane that is placed just above the bottom of the tires, which typically denotes the ground plane to use in your analysis:
I would recommend setting your external flow domain box equal to (or slightly higher than) this plane and re-running the operation.
Furthermore, you may then run into issues with the mesh where the ground plane and the tires meet at a tangent. This is commonly alleviated by adding a small platform below the contact patch as described in this thread.
Let me know if those recommendations resolve your meshing error.