How do I fix OneNote not allowing me to rename a section or page on a Mac?

Ensure that Notebook is synced
Go to the Notebooks dropdown list and make sure the notebook that you want to rename is synced. If it is not synced, click the Sync Now button.

Close and Restart the Notebook
Close the notebook by clicking the File Menu and then Close Notebook. Restart the Notebook by selecting it from the list of notebooks and clicking the Open Notebook button.

Rename notebook from the ‘Notebooks’ dropdown menu
It should now be visible on the Notebooks dropdown list. Click the Rename Notebook button and select the new name. You should then be able to see the changes to the name.