Negative power in a resistor? Where did I go wrong?
The load is not a resistor. If it were a resistor, there would be no phase difference between the current and voltage waveforms.
In general, a negative power is possible --- it means the load has a power source within it and it is delivering power back to your source rather than taking power from it.
So I don't think you have approached the problem incorrectly.
But I'm not sure you you got your numerical result, I got
$$P = 120 \times 20 \times \cos 150^\circ$$ $$P = 120 \times 20 \times -0.866$$ $$P \approx -2078\ {\rm W}$$