In case you overlooked the difference in ttokoros answer - you missed to type a multiplication sign between "w" and the opening parenthesis. Omitting this multiplication and simply typing "w(..." means that you a referring to a function w, which of course is not defined. Hence the error message.
Prime offers a rudimentary error trace facility.
If you type V(3ft)= you get the error saying that it must be a function:
If you now switch to the calculation ribbon and use the error trace facility, you are pointed to the place where the error really occurs:
Unfortunately this does not work from the error you got when you tried plotting V.
