An installed version of JRuby is an absolute pre-requisite for installing propane, but you should install java first.

Installing jruby-launcher

It is recommended that you install the jruby-launcher (for optimised performance and suppression of illegal reflective access by jruby warning).

You should set the JAVA_HOME environmental variable before installing gem, and use JRuby version you want to use with propane (preferably latest available).

Debian linux

First set the JAVA_HOME environmental variable, you can do this by editing the ~/.profile file:-

echo "export JAVA_HOME=/opt/jdk13 >> ~/.profile" # wherever

then

source ~/.profile # to use the edited ~/.profile in current shell
sudo jgem install jruby-launcher # NB: you do need sudo access here

Other linux

First set the JAVA_HOME environmental variable, you can do this by editing the ~/.bashrc file:-

echo "export JAVA_HOME=/opt/jdk13 >> ~/.bashrc" # wherever

then

source ~/.bashrc # to use the edited ~/.bashrc in current shell
sudo jgem install jruby-launcher # NB: you do need sudo access here