By default, the cause is one of the following:
- The class was closed manually and must be reopened. See also: Close and reopen a class or Close and open a class for new registrations
- A «Registration begin» has been set for the class and the registration date is earlier than the registration start date. More: Class settings explained
- The start date of the class (field «Begins at» in the class settings) is further in the future than the desired registration date, i.e. the class has not yet started at this time.