One last time:Replace that pixel with a new one.Does your DVM measure +5V when you put your leads across VCC & VSS?Did you try a different I/O pin on the Arduino?
As Larry has said, the lack of the capacitor is not the cause of complete failure to work, but it is specified and indeed as is so often pointed out on this forum, a bypass capacitor is specified for every IC used - and these LEDs are of course, just that - ICs. (I first typed "logic IC" but that is in itself, misleading as the requirement is no less critical for analog ICs!)I suppose two things remain: I am not familiar with FastLED, but Larry has verified your code.OK, just disconnect the wires from the PCB and carefully connect them to the correct pins on one Pixel.Your PCB is in any case completely useless for the lack of the 0.1 µF chip ceramic capacitor for each chip and the wrong power traces.
I am aware that a pour will lower impedance, so I am working on widening the power traces that run underneath the LEDs.
If your PCB is 16 X 16 pixels it could take as much as ~15 amps.For powering things, suggest your have 4 rows of pixels in parallel, therefore you will have 4 such groups.
Should I get a 20V DC power supply and run the 4 groupings in series to split the voltage across the 4 sections?