Thanks for tagging me in @jousefm!
“Error: Fluid interface should have exactly one unique pair of master and slave entities.”
^^This error occurs because the CHT solver requires individual faces to be mapped onto corresponding faces of adjacent solids. Hence the interface definition should have a single face as Master and its counterpart in the adjacent solid as Slave (topo sets might not work). This might be too much overhead to define all the interfaces.
In this regard kindly note that the interfaces are automatically defined when the mesh is generated and hence interface assignment is not required at all. This can be seen from the mesh log.
Please try to define all the boundary conditions, leaving alone the interface definitions and proceed with the simulation run.
I hope this clarifies!