System.getProperty() is a Java construct. That means, that if you want to call this function in your JavaScript you need to specify the whole packagename infront of the class. Resulting in a statement like this:
java.lang.System.getProperty(…);
If you leave out the packagename, the engine does not know which class to use and throws the error, that it cannot resolve the identifier.
So maybe you should try adding the packagename to your statement. I am not hundred percent sure, if this can only be used in a JavaScript or in your expression as well. Otherwise there is also the option to use a script as an input parameter.
Hope this helps
Regards
Michael