OneNote won't sync and updates don't work.

Hello PeggyRansom,

Welcome to the Microsoft Community!

Thank you for providing detailed information about the sync and update issues you're experiencing with OneNote on your Mac. The version of OneNote you're using (16.16.27, released in October 2020) is significantly outdated, which is likely the primary cause of the current problems. Microsoft maintains a clear support policy for Office on macOS (including OneNote): the software is only guaranteed to receive feature updates and security support on Apple's latest three macOS versions. Your outdated OneNote version may no longer be compatible with current sync server protocols or security requirements, resulting in failed synchronization and prompts stating an "upgrade required" for updates.

To resolve this fundamentally, you'll need to uninstall your current OneNote and install the latest available version compatible with your macOS version. First, please check your Mac's operating system version (click the Apple icon in the top-left corner > "About This Mac"), then refer to Microsoft's official support policy: Upgrade macOS to continue receiving Microsoft 365 and Office for Mac updates - Microsoft Support

You must visit Microsoft's official Office for Mac release history page to locate and download the specific OneNote installer for your macOS version: Update history for Office for Mac - Office release notes | Microsoft Learn

On this page, you can search by date or version number to find the corresponding installation package (16.88, 16.77, or 16.66) listed above. The App Store may only offer the latest version, which could be incompatible with your system. Please use the offline installer package (.pkg) downloaded from the release history page.

Before installing a compatible version, you must completely uninstall your problematic OneNote (16.16.27) to prevent residual files from interfering with the new installation. Follow these recommended uninstallation steps:

Quit OneNote: Ensure OneNote is fully closed. Right-click its icon in the Dock and select "Quit," or use "Activity Monitor" (Applications > Utilities) to force quit all Microsoft OneNote-related processes.

• Uninstall the main application: Open Finder > Go to the "Applications" folder.
• Locate Microsoft OneNote.
• Drag its icon to the Trash.
• Alternatively, select it and press Command (⌘) + Delete, or right-click and choose "Move to Trash."

Remove critical residual files (essential step): Older versions often leave behind configuration files causing issues with new installations. Manually delete them by:

• Opening Finder.
• Clicking "Go" in the menu bar > "Go to Folder..."
• Entering the path: ~/Library/Containers/ and pressing Return.
• Finding the folder named com.microsoft.onenote.mac in this location and dragging it to Trash.

While waiting for the client to sync, we strongly recommend immediately opening your browser and accessing the OneNote web version using the same Microsoft account. Verify whether your notebooks and content exist intact in the cloud. This helps confirm:

• Your note data remains secure on the server-side.
• The sync issue was limited to your old Mac client, not your account or server problems.

If the web version shows complete content, successful synchronization with the new client is likely just a matter of time (though individual notebook conflicts might need resolution). If content is missing on the web version, please check whether your OneDrive web version storage space is full.

Your issue primarily stems from using an unsupported outdated software version. Following these steps to uninstall the old version, install a system-compatible version, and verify your data status via the web version should effectively restore OneNote's sync functionality. Please prioritize using the Microsoft official release history link we provided to obtain the correct installation package. Thank you for using Microsoft products! 

Best Regards,

Ami | Microsoft Community Support Specialist