I find posts online going back more than a decade explaining that this command now provides text results rather than numeric version results. Your scripts are expecting a number, so they fail. Pulse needs to change these scripts.
Linux Standard Base is a project meant to provide commonality between distributions. I suggest using their lsb_release command to find a numeric "Debian" version number. For instance, using the command on my (Debian->Ubuntu->)Linux Mint 18.3 machine:
[email protected] ~ $ lsb_release -v
LSB Version: core-9.20160110ubuntu0.2-amd64:core-9.20160110ubuntu0.2-noarchrinting-9.20160110ubuntu0.2-amd64rinting-9.20160110ubuntu0.2-noarch:security-9.20160110ubuntu0.2-amd64:security-9.20160110ubuntu0.2-noarch
You can strip the "core-9" value to achieve Debian version 9, I think. At least, I hardcoded version 9 into your scripts (/var/lib/dpkg/info/pulse.postinst and /usr/local/pulse/PulseClient_x86_64.sh), and I got the product to install and work just fine. You can see more info here about using this command.
Please, please, update your scripts. I do have users at home (me included) who need to remote into work, but your product as provided does not function.