AppImage provide everything that is needed to get executed. It works like a .exe on Windows and does not need to be installed. See here for our latest Nitrokey-App.AppImage file. After downloading the file, you need to make the file executable. You can do this by right-clicking on the file and choosing "Properties". Look for the option to make the file executable there.
Note: There is a problem with the current Snap package. Please make sure that scdaemon is installed by typing
apt-get update && apt-get install scdaemon in a terminal and follow the instruction below.
Snapd is already installed on Ubuntu, for distros like Arch, Debian, Fedora, or Gentoo you need to install snapd first. As soon as you have Snap installed you can open a terminal and type
sudo snap install nitrokey-app --devmode
In case you get the error "failed to create user data directory. errmsg: Permission denied" during starting the Nitrokey App, it may is because your home directory is encrypted. In this case, apply one of these fixes  .
Download the Flatpak file and install it with "flatpak install nitrokey-app-1.4.0.flatpak" (filename may differ).