JIRA User cannot add Watchers to an issue
When one or more users try to enter a user's name in the Watchers field for an issue, the auto-complete functionality doesn't operate.
This user will also be unable to use the @mention facility to mention users in comments.
This is caused by the user being denied the Browse Users privilege.
Ensure the user involved has the Browse Users global permission, or is in a role that includes that permission.
More information can be found at Managing Global Permissions.