The Program Review module is equipped with a Check-In/Check-Out feature.
When a user enters Edit Mode of a requirement, the item is checked out, allowing only that user to edit the requirement at that time.
If you receive a message stating another user has the item checked out, contact that user and ask them to check the item back in if they are finished. They can do this by going to Options > Check-In.
Site Administrators can override user Check-Outs if necessary.