Yes, you can list a unit even if it is currently occupied. You will see the appropriate status icon on the unit card.
Once you have moved a tenant in and a lease turned active, it will automatically unlist this certain property. However, you are welcome to relist it if needed.
To list an occupied unit go to the Leasing → "Listings" page from the left-side menu and press “List unit +“ on the unit card you would like to list (you can check the "Unlisted" display option).
Fill in all the required information and post the listing.