It would be easier for people to help you solve this problem, if you added some example images of the UI that you can see in front of you 
I’m not sure where or how you have added the {{username}} syntax - It can be used in a number of different areas of the app.
If you’re trying to use this syntax in the scripts, it won’t work like that and you will either need to use pm.environment.get('username') or pm.environment.replaceIn('{{username}}') to access the value.