Yes, you can but only for Single-session Courses.
This can be a strategy to pull more interested parties over the fence.

On the Course edit page, add your own coupon code in the field "Free Course Coupon Code".

Case:
Imagine you've set a coupon code on your Single-session Course, i.e. a string of characters without spaces. You activate this Course and then promote it in your social circles showing the link to the Course and the coupon code. A potential customer sees your social post and decides to click the link. They arrive at your Course page and there they click the "Book now" button which sends them to the Course booking page. There they have to "click the plus icon", at the top right of the page, and enter the coupon code you posted. When they finalise the booking process, as per usual, there will be no payment and their Course becomes immediately "active" which means that they can now book a Session for this Course from one of your Available Sessions.

Note i: The Student can book this Course for free only once. If they've bought this Course before and paid for it, they can still book this Course for free by using the correct coupon code.
Note ii: if you don't want to offer this Course for free any longer, simply remove the "Free Course Coupon Code" from the "My Activated Courses" page.