If you have a student account and your Dashboard shows all topics as locked, this is because you have already started a topic.
You can only have one active topic at any time. To start a new topic you can either complete the topic you have already started, or you can cancel the progress for the current topic and start a new one.
Remember that completing a topic includes finishing all learning steps, earning 30 XP in the relevant Dialogue Space, and completing the Progress Tracker for that topic. You can follow your progression for a topic by looking at the Progress Bar in the Dashboard: