The first error is that you forgot the function arguments when you defined rt. That way you defined a simple variable, not a function (thats what the error message tries to tell us).
The second error is that you tried to access the columns of S by typing S^(0), etc. You have to use the column selector (either from the menu or by typing Ctrl-Shift-C).
A third problem occurred due to Prime infamous auto labelling. I had to manually label the highlighted variables in the plot to be "variable".