You must be an admin of the Facebook Page to turn on syncing. Detailed instructions on how to link Facebook Page with WhatsApp Business account can be found here.
Please note that the syncing only works one-way: from Facebook Page to the WhatsApp Business app.
All the changes made on the Facebook Page will automatically reflect on the WhatsApp Business app but the vice-versa isn’t true. For example: if you try to change the business hours on the WhatsApp Business app, you’ll get a warning message that the syncing will stop. If you proceed to stop syncing and change hours, then the WhatsApp Business account will reflect new hours, and the Facebook page will continue reflecting old hours.
To enable/disable the sync, you must have:
Note: Once the sync is enabled, it will work on both web and mobile.