The Populate button is missing for multiple users, including a user who has admin access.
Go to an open Timesheet
Click 'Configure' link
Copy time entries from previous timesheet is unchecked
Check 'Copy time entries from previous timesheet'
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.