One suggestion, Mechanical automatically created a Contact region in Meshing, which created an extra boundary condition in Fluent.

Go back to DesignModeler, and make sure the solid and fluid bodies are in a Multibody Part by using Form New Part and set that part to Share Topology.

Then go back to Meshing and delete the Contact.

Then reread it back into Fluent.  It will notice a BC is missing. Follow the directions to resolve that.

What you have works, it's just more complicated that it needs to be.