At this rate, I’d have more chance to build from source directly, although
the first time I’d do it.
You do not have to.
https://github.com/Nitrokey/nitrokey-app/releases/download/v1.4/nitrokey-app-1.4.0-experimental.deb
can be installed on Trisquel 8: with GDebi (installed by default),
There is no "nitrokey-app" package for Ubuntu 16.04 Xenial Xerus in
http://ppa.launchpad.net/nitrokey/nitrokey/ubuntu/pool/main/n/nitrokey-app/
I would try Ubuntu 18.04 Bionic Beaver's packages (there is little risk here:
the PPA only provides additional packages; it does not upgrade
Show us the line for the PPA in /etc/apt/sources.list.d/
