Controlling settings, permissions, and membership within Teams and channels
Hi Marié,
Per my tests, you may need to click on the Notes tab in each channel to finish the creation process, after that, you can switch to the General channel and click on the Class Notebook tab to check the sections from the Collaboration Space.
Reference: Use OneNote Class Notebook in Teams (The "Use Class Notebook in a channel" section)
Hope this helps.
Best Regards,
Hugo