The Nitrokey App is included in Ubuntu's repositories since 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
To get the most recent Nitrokey App you can add a separate repository which is maintained by Nitrokey.
Please execute the following in a terminal to add the repository and install the Nitrokey App:
sudo add-apt-repository ppa:nitrokey/nitrokey sudo apt install nitrokey-app
Alternatively, you can use the snap package. Please see here for more information.