In order to analyze a model, the mesh must be complete. If there are any holes in the mesh, they must be filled.
Use the 
Fill Hole command to fill holes and gaps in the mesh with triangular elements. Select a sequence of connected nodes. This is the boundary to be filled with triangles. The software automatically assigns the properties of the surrounding elements to the element created.
Note: The Fill Hole command works on triangular elements, and cannot be used on tetrahedral meshes.