It is important to separate between the Java layer and the script layer. The generateScript() method should only contain the scope of script writing - for more information you can read the articles provided in getting started.. When creating a variable, it is not visible in the polyScope. Also you need to assign a value to the variable in polyscope and then refer to the variable in your generateScript() method in program node to use it.
Could be something like this Installation node::
assign a value in the polyScope (robot UI):
and it should be possible to refer to it in the program node:
this is just an example.