So a common problem can be, that the surface deform on the main mesh cannot be bound to the proxy mesh due to concave polygons. It will look like this:
The cause for this is that one or more face, is not convex, but concave meaning one of the outer corners has a angle of smaller than 180 degrees.
Solution 1: Find the problematic face manually and adjust its position, so it will become convex again, and not pointing inwards like so:
Solution 2: Select all the vertex from the proxy in edit mode, and then go to Mesh -> Cleanup -> Split Concave faces. This will split all the concave faces into triangles.
After you fixed it by one of the 2 methods, it should be possible to bind the main mesh to the proxy object without getting the concave polygon error.