Please follow the steps below

1. Go to the general configuration page (Settings -> General configuration)
2. Make sure that the check box Allows automatic calculation on the number of days is checked
3. After that edit working hours, break time, weekend to consistent with your organization
4. If you want the the employee can change the number of days. Please make sure that the check box Allows edit the number of days is checked
5. Click the Update button at right of the row