When you connect a Linux client computer to the server, you will sometimes see that the status states connected and the mouse hovers over some parts of your client screen, but you do not see the mouse pointer. The common cause is that your Linux build is Wayland on the desktop.
How do you find out if you are using Wayland?
Open the terminal, and run the following command:
> loginctl show-session "$XDG_SESSION_ID" -p Type
If the system responds "Wayland", Synergy will not work. We have Wayland support in our roadmap but we cannot provide an exact ETA at this time.
For now, you will need to switch to a different build such as Xorg.
This article may help for Ubuntu: