1. Check if the notebook is protected with a password. If the notebook is password protected, try unlocking the notebook with the correct password.
2. Make sure OneNote is up to date and if not, update the app.
3. Make sure that the operating system you are using is up to date and if not, update it.
4. Determine whether the file you are trying to upload is supported by OneNote on a Mac. You can find a list of available file types under the list of file types supported by OneNote for Mac .
5. If the file or attachment is still not being accepted, clean the notebook cache by following the instructions here.
6. If the problem still persists, you might need to reset the notebook. To do so, locate the notebook folder on your Mac, delete the entire notebook folder, and then reopen the notebook again.