Hi username2
There was a previous user that was unable to install rar.
rar is only available from non-free. (I am able to install rar in Qubes-Whonix 14)
https://packages.debian.org/stretch/rar
However, you should avoid non-free packages:
https://whonix.org/wiki/Warning#Avoid_Non-free_Software
This can be sorted out as per:https://whonix.org/wiki/Support#Free_Support_Principle
1. Check if you have non-free added to your sources.list
cat /etc/apt/sources.list.d/debian.list
The output should be something like this (Whonix 14)
deb tor+http://sgvtcaew4bxjd7ln.onion stretch/updates main contrib non-free
2. If non-free is not added, search “how to add non-free to debian sources list”