Check that you can access your SharePoint Online
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Sign in to Office 365 with your work or school account.
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Browse to your SharePoint Online location.
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Select the Document folder or Library that you are trying to sync.
If you don't see the sync option, or you see a screen that says "Which library do you want to sync?" then the site has not been configured for syncing, and you should contact the site owner.
Note: Page or site owners can edit via Library settings > Advanced settings > Offline Client Availability.