I visited the plexydesk website and received the attached graphical message stating version 0.7 is not available on the GNU/Linux platform. This my be the source of your problem.
do you understand how to modify the /etc/apt/sources.list file to include the repository in question in apt-get?
for help with compiling from github
More than you were hoping for, I know, but . . . so go third-party Linux packages!