Jira Software - When search in Watcher field, user does not appear to be selected

Question

Tried to add colleagues as watcher into one of my Epic. Some colleagues are available and some don't. Upon checking, the one that is not available

1. Has Jira Software access
2. An Administrator via Group (in project setting > people)
3. Has their own Epic/Task in the same project

Anything else that I am missing to check?

Nor Hisham Nordin · Answer

Correct. I have no idea what other setting that I need to configure to be able to add the user as Watcher.

Alex Koxaras -Relational- · Answer

Yup, that's the one.

So when you search by his name on the watchers field, you can't seem to find him, correct?

Nor Hisham Nordin · Answer

This should mean they have the browse project permission right? 
Under Project > Project Setting > Permissions