Using a backport
From Andreida
Add to the sources files:
vi /etc/apt/sources.list
example for Bullseye (Debian 11), add to the end of the file:
# backports deb http://deb.debian.org/debian bullseye-backports main contrib non-free deb-src http://deb.debian.org/debian bullseye-backports main contrib non-free
apt-get update apt-get install rsnapshot # or whatever you needed
