File constant/polyMesh/boundary must be edited. Remove empty patch (bkg_mesh_walls), rename non-empty patches and edit their properties^13.6:

3
(
    rotor_inner_wall
    {
        type            mappedWall;
        sampleMode      nearestPatchFace;
        sampleRegion    rotor;
        samplePatch     inlet_wall;
        offset          ( 0 0 0 );
        nFaces          15075;
        startFace       1702983;
    }
    stator_inner_wall
    {
        type            mappedWall;
        sampleMode      nearestPatchFace;
        sampleRegion    stator;
        samplePatch     outlet_wall;
        offset          ( 0 0 0 );
        nFaces          59442;
        startFace       1718058;
    }
    outer_wall
    {
        type            wall;
        nFaces          95051;
        startFace       1777500;
    }
)

Patch rotor_inner_wall is adjoining the rotor region and is of type mappedWall because heat transfer from the fluid to the solid and vice versa is assumed. Patch stator_inner_wall is adjoining the stator region and is of type mappedWall. Patch outer_wall is a standard wall.