Most likely you are not being able to select users as you lack the Global Permission to Browse Users.
Please look at the permission at System/Global Permissions https://<YOUR-SITE>.atlassian.net/secure/admin/GlobalPermissions!default.jspa, and you probably can see that the permission is only granted to the jira-developers group which you are not a member of.
To fix this, I would suggest for you to either add yourself to the jira-developers group or add the group which you are a member of to the permission. You would then be able to search for users.
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