If you see a red X in front of your course name on the Main tab, the course link is not available to students.
To make your course available, click directly on the red X. It will change to a green O, and the course link will then be available to students. To make the course unavailable, click the green O to change it back to the red X and make the course unavailable.
You can change it back to unavailable by clicking on the green O.