Sync chat history is that you can see your chat on multiple devices at a time. Viber also provide this feature to its users.
For sync chat history to desktop, first of all you make sure that your mobile Viber app is of latest version. Then install Viber on your computer or laptop and open application.
For the very first time, the sync button popup on desktop. Tap on the sync button.
Then a popup appears on your mobile. Click on Approve button to allow syncing between your mobile phone and desktop. Keep open Viber app on you mobile until sync completed.
By following this process, you can sync your chat history to your Viber for desktop.