If you cannot edit a notebook, you may be trying to edit a read-only section. Try navigating to Collaboration Space or your private notebook where you have edit access.
Student Notebooks — private notebooks that are shared between each teacher and their individual students. Teachers can access these notebooks at any time, but students cannot see other students’ notebooks.
Content Library — a notebook for teachers to share course materials with students. Teachers can add and edit its materials, but for students, the notebook is read-only.
Collaboration Space — a notebook for all students and the teacher in the class to share, organize, and collaborate.
For more information, visit Getting Started with the OneNote Class Notebook: A Walkthrough for Teachers.