User is not available in the User mention list.

andrew sutton

One answer seems to be missing from the suggested problems.

This is that the user needing the mention does not have the permission to see the project.

 

In Project > Admin > Permissions, ensure that the browse Project permission contains something that the missing user mention is within.

unknown

As you mentioned, check if that user has the "Browse Project" permission according to your project permission scheme. It maybe tied to userID/role/group.