Here are the steps you can take to fix OneNote not allowing you to rename a section or page:

1. Close all OneNote windows and restart the application.
2. Right-click the section tab and select Copy.
3. Right-click another section tab and select Move or Copy.
4. Select the section to which you want to copy the renamed section tab, and then click Copy.
5. Right-click the section tab of the original copy and select Rename.
6. Enter the new name.
7. When you’re done, close OneNote.