The Populate button is missing for multiple users, including a user who has admin access.
This can be due to Copy time entries from previous timesheet being unchecked
To check if this is the cause:
Release: All Supported
Check 'Copy time entries from previous timesheet'.
To make this change for one user:
Below are the steps to make this change globally
1. Go to Administration>>Project Management>>Timesheet Options
2. Check 'Copy time entries from previous timesheet'
3. Click Save
4. Click 'Apply to All Resources'
This will allow all users to see the populate button.