To create open shifts by department, you need to first create 'Departments' for your employees

Here's how you can do that:

Head to settings panel --> People --> Departments page. 

Click on the create button and "Department" will show up as a shift attribute while creating any new shift. 

How to create open shift:

Once the department is created, please head to the Shift Management --> create shifts --> Open Shifts section to create open shifts 

Click on "Show" next to employee filtering and pick "Department" as aa attribute category. Pick the department from the dropdown menu and click on "Create open shift"