Several ways to fix common sync errors that you will experience while using OneDrive.
1. Make sure OneDrive is open
2. Make sure you are logged into OneDrive
3. Pause and un-pause syncing
4. Un-sync and re-sync SharePoint document libraries
5. Log out and log back in
6. Reset OneDrive (https://support.microsoft.com/en-us/office/reset-onedrive-34701e00-bf7b-42db-b960-84905399050c)
7. Uninstall and re-install OneDrive.
