Hi liara,
Issue:
The installation of SABnzbd (e.g. 'box install sabnzbd') fails silently and is further reported as 'installed'
This issue is new to Debian 9 (Stretch) distro.

Install Sabnzbd Linux Mint

Troubleshooting:
I reproduced the installation manually (from sabnzbd.sh) and the failure is at this command:

Sudo apt-get install sabnzbdplus-theme-mobile. How To Start. And that’s it. To start the program, just execute: sabnzbdplus. If this is the first time you run SABnzbd (or, more exact, if there is no sabnzbd.ini in ~/.sabnzbd/), SABnzbd will start the wizard which lets you fill out the needed parameters. Install PAR2 Multicore Sabnzbd Debian + Ubuntu Linux. Par2tbb is a project which enables multicore par2 processing for par2 files. Par2 files are used to repair incomplete rar files typically found on usenet. Being able to repair rar files using multiple threads using all of your CPU cores can signficantly increase the time taken.

Additional info:
In Debian stretch the package python-software-properties is not available, it seems that it is replaced by python3-software-properties which is automatically installed as a dependency to software-properties-common.

Reference:
https://tracker.debian.org/pkg/software-properties
https://packages.debian.org/src:software-properties

Install Sabnzbd On Ubuntu

Proposed solution:
In /scripts/install/sabnzbd.sh, replace line 30,
replace the line
apt-get -y install software-properties-common python-software-properties >/dev/null 2>&1
with
apt-get -y install software-properties-common >/dev/null 2>&1

This solution is fine for Debian 9 (Stretch) but I made no regression test for other distro. So, this may need to be further tested for other distro.