A knowledge base article about I just added the course but I can't log into bCourses provided by the UC Berkeley IT Service Hub - Knowledge Portal
bCourses enrollments are refreshed daily from the Student Information System at around 6AM. Therefore, students who just registered for the course on CalCentral, or instructors who have just been added by their department scheduler as the official instructor of a course, will get access to the course site on the following day.
If needed, anyone with the Teacher role in the course site can give immediate access by manually adding students or other instructors. Please see How do I add users to my course site? for instructions on how to add people to a course site.