Unable to install Imunify360 in Plesk: The value ‘buster-backports’ is invalid for APT

Symptoms

Unable to install Imunify360 on Debian 10 with the following error:

E: The value 'buster-backports' is invalid for APT::Default-Release as such a release is not available in the sources
iptables >= 1.8.5 required on Debian 10

Cause

Unsupported backports repository is not configured at the server.

Resolution

1. Connect to the server via SSH

2. Add backports repository:

   # echo "deb http://archive.debian.org/debian buster-backports main" > /etc/apt/sources.list.d/backports.list

3. Install iptables from the added backports repository:

   # apt-get update && apt-get -t buster-backports install iptables