Body Fitting doesn't like submeshes

Hello,

I have a geometry which is entirely cartesian. It consists of cuboids. One large base, with maybe 2-5 smaller ridges all running parallel at the top. I would like a finer mesh for the smaller ridges at the top, whereas for the large base I do not need as fine a mesh. Previously I did this using Gmsh 1D-2D-3D which created sub-meshes without any issues but I would like to utilize the fact that using ‘Body Fitting’ I can create a finer mesh profile (X-Z) with a slightly coarser mesh in the Y -axis.

When I try to create a sub-mesh of the ridge I receive this error saying that: No sense in creating a sub-mesh ignored by global algorithm (“Cartesian 3D”)
image.

My primary aim to create a (hexahedral) mesh which is a) finer in the X-Z profile and is longer in Y, and b) can assign a sub-mesh which has a finer mesh with respect to the rest of the geometry.

If you have any understanding why this error arises or perhaps another method that I could use please let me know!