The Project Resource Planning functionality is not enabled by default. A System Administrator can enable it for your organization.
To enable the functionality:
- As a System administrator, navigate to Settings > System Settings.
 - Expand Resource Management.
 - Select the Enable resource planning checkbox.
 - Click Save.
 
...this appears to be in Resource Management, not "Work Management Settings".
what does it mean when enable resource planning checkbox is grayed out and ON