Some mesh operations cannot be performed in Blender if the mesh is non-manifold. To view the non-manifold elements of the mesh:

Switch to Edit Mode. The number of non-manifold elements in the mesh can be seen at the top where the number of vertices, edges and faces is displayed. For example, a non-manifold mesh of 14 faces, where 2 faces are non-manifold would be shown as Fa:2-14.

Strangely, Blender only displays and selects the non-manifold elements in Vertex or Edge select modes. It does not work in Face select mode. So, choose either the Vertex select mode or the Edge select mode. Next, choose Select > Non Manifold. You can also do this by pressing Shift-Ctrl-Alt-M. The non-manifold vertices or edges are now displayed in orange and are selected for use in further operations.

This very very helpfull !!!!!!!!

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