Debian jessy installation
unfortunately Debian Jessie is not supported as the Qt library shipped with it is too old. Veyon requires at least Qt 5.5. If you manage to install Qt >= 5.5 on your system you can build Veyon binary packages on your own as described at https://github.com/veyon/veyon#veyon-on-linux. Otherwise you'll have to update to Debian Stretch or newer.