You've set your task up for 2 channels ai0 and ai1. You are getting N channels thus a 1-D array of waveforms that will contain 2 waveforms. Then you are indexing that array to get 2 Voltage display, and a 3rd waveform that you are feeding to frequency.
That waveform is empty because you never acquired it!
I imagine you probably want to do the frequency off of one of the two voltage waveforms you acquired.
PS. You don't need the constants 0, 1, 2 on the Index Array. It will give you those indices by default.
