If you want to grant permissions to an employee to ONLY 'Add and duplicate Job Openings' you also need to ensure that they have visibility over any other Job Opening, otherwise they will not be able to add a new Job Opening from scratch.
The system works this way because of the duplicating part of the permission. If they don't have any visibility over another Job Opening then this won't work as intended and they will not be able to create a Job Opening or to duplicate.
What you can do as workaround is to either assign the employee as part of the Hiring Team for an already existing Job Opening OR to create a new test Job Opening just so you can add them as part of the Hiring Team. Following this workaround, the employee will be able to add and duplicate any new Job Openings.