A few ideas:
Has the class started yet? Class times are shown in Eastern time, and they become joinable 10 minutes before the class begins. If you don't see a "Join Class" button, the class session has not started yet.
Is the class full? Classes have a maximum student count, though the limit is quite high, so this might be surprising
Are you super late? Classrooms are locked automatically 10 minutes after the session starts to prevent disruptions. Be sure to join before the lock window!
Are you using Google Chrome or Mozilla Firefox? Classes do not fully support other desktop browsers at this time.
Do you meet the prerequisites? Some classes require prior completion of foundational courses or other requirements before registration.
If none of those solve the issue, please send us a note to let us know about the issue, and can fix it ASAP!