Problem:
My company recently completed a SharePoint 2010 to 2016 migration. With the migration came the use of HTTPS security, so my OneNote notebooks stored in SharePoint would no longer sync. All of my notebooks displayed an error of Not syncing.

Here is how I fixed the issue:
First, make sure all of your notebooks are in SharePoint.
In OneNote, click on the File tab.
Locate the first notebook you want to update.
Next to the notebook name click Settings, then Properties.
In the Properties window click on Change Location…
Copy the URL of your SharePoint document library.
Paste the URL into the OneNote Chose a sync location… window.
Select the folder you want to sync OneNote with.
Click OK, a message box should appear saying the item is syncing.
Give it a minute and you should be set.