First, the inlet boundary condition is applicable only to faces (2D) only and not cells (3D). From your image above, it looks like 2 of the circles are in the middle of the domain. It would not be necessary to mark these as inlet (would it make physical sense?). Furthermore, if you do not have a faces exactly at that X or Y plane where you are making these, there will be nothing to select. The mesher likely put some cells that go through these planes and it is unlikely that you have a coincident vertical plane at these unless you have taken VERY special care.

That said, what you may want to do it open Fluent Meshing. In the Splash screen choose Meshing not Solving. Here you read in your mesh file. Under Boundary-> Zones -> Separate, a window will open. What you can do is create Local Regions over the desired faces and then Separate the walls. Then using Boundary->Manage you can give these faces a new/more appropriate name.