Dear Seyede,

Ara both faces touching? Maybe they share one of their edges? If so, check that you do not have created that edge twice such that you have two edges superposed.

Also, you may check whether you have virtual entities in that face (the face itself, or any of the edges or nodes may be virtual, and you such try to avoid having virtual entities which may be troublesome to handle).

Hope this is useful, but with the available information I cannot think of much more (plus, it's been a while since the last time I used gambit; cannot you use another mesher? there are many meshers that are much better)

Jose

what type of mesh did u make? in rectangle  geometry use hex-map mesh .. if the geometry is made from subtraction of another geometry than has any cut of corners this will not work. For Cooper make one face and then use cooper imprint to other face and than mesh volume.