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I've encountered this error before. Most of the time, it is telling you the truth. That is, you probably tried to execute a Boolean difference operation in 3D where at least one of the items involved was not a solid. One common error is for the user to unintentionally create a surface (not a solid) in 3D by extruding or revolving a curve (not an area) in 2D. The extruded or revolved object may look a lot like a solid in the graphical display, but it is really just a 2D surface, and then the code won't let you execute the difference operation on it. In that case, all you have to do is go back and make sure you extrude or revolve only true surfaces in 2D (to become solids in 3D), not just the edges (which become surfaces).

Another possibility is that your overall geometry is not actually being correctly resolved by the code, which can happen if you have small errors where points, edges, or surfaces were intended by you to be coincident, but were actually only very close, but not coincident. So take another look at your geometry and be especially wary of places where edges or surface come together tangentially, or where points are very very close together. If that is the case, modify the troublesome geometry so that the points, lines, or surfaces involved become either truly coincident or more widely separated.

Thank you very much.

But I did it Comsol 4.4 and it did not get any error. I mean the procedure was the same for both but I got that error in 4.3.
by the way thanks very much for your detailed response.