I have a custom object (case) with look up field(Assigne agent) on user , lookup filter ( asignee agent > profile>name equals system administrator). When I create a new case ,I'm not able to look up / select any users on Asginee agent field. Any suggestions are most appreciated.
Looks like there is an issue on this in Lightning. There is an Idea posted on Salesforce, you can take a look at this and probably upvote this Idea
But if you are on classic and still cant see the users, you have to go to Setup > Search Settings (under customize section) > Lookup Settings section > click on the checkbox for user on Enhanced Lookup. This will then allow you to be able to fetch results.