When meshing cannot link faces.
 

This a geometry of a water tube -Attachment 35542.Wen meshing,I want to link the two Attachment 35543，but the gambit prompts：ERROR:Cannot link faces face1,face6 duo to all loops of edges not being specified.That is why? Thank you !

you don't need to link the faces.
I would split both cylindesr at the extremities, then mesh the central volume prior to both cylinders

This is a simple example ,I want to find the find reasons .The problem often display in the complex exmples when I was meshing.

I could reproduce your issue.
I fixed it, by splitting both surfaces.
It is possible that gambit has a problem with a hole in a surface.
Attachment 36163