When simulating a (structural) heat transfer analysis, it is not possible to define the conductivity between contact faces. There are two workarounds you could try:
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Switch to the Conjugate Heat Transfer analysis type. Even though this is one of the Fluid Analysis solvers, it is possible to use only solid materials and omit any fluids. Then you can use the thin layer resistance option, that allows to introduce a virtual layer between the parts that affects the heat transfer between them: What is a thin resistance layer in CHTv2 and how to define it?
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Add a thin layer between the two parts and assign a material according to the resistance you would like to simulate between them. This has to be designed, added to the CAD, and meshed appropriately. For thin parts, make sure to check this mesh refinement method: Partial Thin Mesh