This question already has an answer here:
Imagine we have a wave with a certain frequency. We can calculate the energy of this wave with the equation
$E=hf$, where $h$ is the Planck constant and $f$ the frequency
Now, we can create a second wave, with the same frequency but a phase of $\pi$, so when we add the two waves they cancel out.
However both waves have positive energy, so their energies should add up, giving twice the energy.
In reality, the waves might not perfectly cancel, giving a residual with the added energy of the two waves, but in a pure conceptual design, how can "nothing" have energy?
Or am I missing something?