You tried to plot something that is not available
or doesn't exist.
Maybe you forgot to save something that the
"power contours" needs. Or maybe your plot
selection is just malformed and can't evaluate.
This happens a lot when trying to build up
complex Calculator expressions to plot.
In that case, validating the "chunks" and saving
known good bits in a text editor (because
Calculator expression editor sucks badly, last I
tried) that -can- evaluate, by process of elimination
may lead you to the bit that causes it not to.
Complaining about the result, without a hint
to methods, stands little chance of returning
a better clue than that.