After using the Save document function to save a OneNote Notebook as a file, attempting to open the resulting .onepkg file using on-premises OneNote, most notably in OneNote 2016 and OneNote 2019, the following warning is observed:

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As a workaround to make such a Notebook editable, force the notebook to resync by using the Change Location function within the Notebook properties. After the folder is changed, data will be re-synchronized, and the notebook will be available for editing.