Yes, you can offer a Course free of charge but only for a limited number of times.

On the Course edit page, add your own coupon code in the field "Free Course Coupon Code". This works only for Single-session Courses.

Case:
Imagine you've set a coupon code on your Single-session Course. Your coupon code must be a string of characters without spaces. You activate this Course and then promote it in your social circles with a link to the Course and the coupon code. A potential customer saw 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, to 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. They can now book a Session for this Course by booking one of your Available Sessions.

Note i: The Student can book this Course for free only once. If they bought this Course before and paid for it, they can still book this one for free using the correct coupon code.
Note ii: if you don't want to offer this Course anymore for free, simply edit your Course and remove the "Free Course Coupon Code".