It's a UI bug, if you check the .deb you just downloaded is for Debian, not Ubuntu.
This is the right url for Ubuntu https://symless.com/synergy/download/direct?platform=ubuntu&architecture=x64

Anyways, a lib is missing Package libavahi-compat-libdnssd1 is not installed.

So the workaround is the following

cd /tmp && http://launchpadlibrarian.net/227449616/libavahi-compat-libdnssd1_0.6.32~rc+dfsg-1ubuntu2_amd64.deb
sudo dpkg -i http://launchpadlibrarian.net/227449616/libavahi-compat-libdnssd1_0.6.32~rc+dfsg-1ubuntu2_amd64.deb
Download synergy form the link given above and execute sudo dpkg -i /tmp/synergy_1.10.1.stable_b81+8941241e_ubuntu_amd64.deb