The Nitrokey App is included in Ubuntu's repositories from Ubuntu 18.04 (codename 'bionic') on. You can just install it via Software Center or by typing the following in a terminal:
sudo apt update sudo apt install nitrokey-app
Unfortunately the App versions in the official Ubuntu repositories are often not up-to-date.
More recent App versions (currently only for Ubuntu <18.04)
To get the most recent Nitrokey App you can add a separate repository at Launchpad which is maintained by Nitrokey. This is not mandatory though.
Please execute the following in a terminal to add the repository and install Nitrokey App:
sudo add-apt-repository ppa:nitrokey-team/ppa sudo apt update sudo apt install nitrokey-app
Alternatively, you can use the snap package. Please see here for more information.