If the “Sync” option is not available for a library on SharePoint, it could be a couple different things.
Note: Media and Form/InfoPath Libraries will not sync but you can still open them in Windows Explorer and pin to Quick Access so they will function as a mapped drive would.
Sync button is available in one library but no another on the same site
Go to the library for which the sync button is disabled
Go to library settings
Click “Advanced Settings”
Make sure “Offline Client Availability” is set to “Yes”
Sync button is not available on ANY library on a site
Go to Site Settings and select “Search and offline availability” from the “Search” section
Make sure “Offline Client Availability” is set to “Yes”
Thank you. THANK YOU!