When creating a project with a low-poly OBJ file or when baking the mesh maps using a high-poly OBJ file Substance Painter can't access the material (-.mtl) file. I've double checked that the material file is valid by loading the same OBJ file in Blender and Marmoset and it works fine. I get the same error even for a simple cube with six vertices and a single material.
I suspect that macOS is possibly restricting Substance Painter from acessing the material files.
I'm on macOS Big Sur 115.2 and Substance Painter 7.2.2 on an Mac mini M1.