Sign in to Office 365 with your work or school account.
Browse to your SharePoint Online location.
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.