This is a OneNote sync error, and sync errors in OneNote almost always come down to the connection between the client and the notebook's storage in OneDrive or SharePoint, so the fixes in order of likelihood:

The account and connection first: OneNote syncs notebooks stored in OneDrive or SharePoint, so the error often means a problem reaching that storage, checked by confirming you are signed in with the correct account, that the account has access to the notebook's location, and that other things in that OneDrive or SharePoint sync fine. Signing out and back in refreshes the authentication that a sync error frequently reflects, the simple first step that resolves a good share of these.

The manual sync and status: force a sync, right click the notebook, Sync This Notebook Now and check the sync status, File, View Sync Status or the notebook's sync indicator, which shows which section or notebook is failing, narrowing a whole notebook problem to a specific failing section that a single large or corrupt page might be causing.

The specific section or page cause: a single problematic section, an oversized page, a huge embedded file or a page with a sync conflict, can fail the whole notebook's sync, isolated by the sync status showing which section errors, then addressed by reducing an oversized page, resolving a conflict page OneNote flags or moving content out of the stuck section to identify the culprit. Large embedded files are a common trigger, since OneNote sync has size limits a giant attachment exceeds.

The reset routes when connection and sections check out: closing the notebook, File, Info, Close and reopening it from its OneDrive location re establishes the sync cleanly and for a stubborn case, OneNote's cache can be cleared, the notebook re downloaded fresh from the server, though this is more involved and warrants ensuring the server copy is current first. Most 0x88ffc008 errors resolve at the account refresh or a specific oversized section, so sign out and in, force sync to find the failing section and address that section, escalating to closing and reopening the notebook only if the targeted fixes do not clear it.