This is probably related to Project level security.  To determine this, login as the admin user (MxAdmin unless you have changed it) and see if your reference selector works.  If you see the users after logging in with MxAdmin, you are having issues because the users you are logging in with don’t have rights to manage other users (which means they can’t see the user names).  To see the settings for this, open Security in the Project node.  Click on the User Roles tab and open one of the user roles. If you see something like the screenshot below, a user with this role won’t be able to see other user ids:

Hope that helps diagnose the problem.

I’m not sure the scope of the problem to be solved, but Mike’s answer has been a solution for me in the past. Definitely have used MxAdmin to test out userRoles/make sure drop downs are actually populating (and filtering correctly).

One solution I’ve used when needing a specific list to populate in a drop down where something other than security has been the issue – either populating by microflow (so your microflow would essentially retrieve and return your list of users), or utilize the AutoCompleteForMendix widget (if that’s a functionality you’d like to implement as well). 

I would definitely start with your security roles though. Also, this may sound silly but make sure you have users! If you’re in a clean environment and haven’t added any yet, that could be it. 

Hi Shivabshu – have you tried to add a dummy attribute to the AllUsers entity? Or to use System.User directly?
I btw typically use Administration.Account which is inherited from system.user anyways.