I can't access the next session!

It may happen that the end of your previous session has not been properly registered.

Make sure you go to the very end of the session to make sure it is accounted for. You should see a message congratulating you at the end of each session. 

If it is still blocked, you can "skip" to the next session

You can find out how to do so in this article: How can I skip to the next session?