You need a working CUPS setup with a compatible printer set up as the default printer and the LPR daemon installed on your server.
In your Instance, simply navigate to »Apps«, choose the category »Tools«, find the Printer app and enable it.
On Debian, you can install the packages `cups` and `cups-bsd` and configure a default printer in CUPS (please refer to the CUPS documentation on how to do that). You can verify the installation with `lpstat -p` and `lpstat -d` after you configured CUPS correctly.
To install the app itself on your instance, simply navigate to »Apps« in your Nextcloud web interface, choose the category »Tools«, find the Printer app and enable it.