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.