Some courses are only open to certain students. These will usually require something called an Enrolment Key to access.
An enrolment key is just a short code that you will be asked to enter before you are allowed to enrol on certain courses. It's usually only a few characters or digits long and will be shared with you in advance by the course tutor or organiser.
The course page will usually state if a course is open to everyone or requires prior registration or is limited to certain individuals or groups.
If you have been accepted on a course that requires an enrolment key and have not received it when the organiser said you would, please contact the organiser in the first instance.
If you have trouble getting in touch with the organiser, please submit a query here.