Problem installing RATT
Usually the home of a program is where the executable is. In order to make the system aware you need to include it as a variable when the terminal is run.

So you need to add it to the file at ~/.bashrc of your computer (if you are using bash as shell [you can check it with echo $SHELL]). You need to add :

export RATT_HOME=path/where/RATT/is/installed/or/downloaded


As terdon says in a comment below:


  Note that echo $SHELL just prints the default login shell of the
  user, and not necessarily the shell that is currently running.  Also,
  ~/.bashrc is not read by macOS bash which starts login shells by
  default. In any case, this sort of global variable should be in
  ~/.profile, not .bashrc. For more of this sort of detail see
  Sequence of scripts sourced upon login and How to correctly add
  a path to PATH.