All the PCs I see today have TWO power cords from the PSU to the motherboard. Later you mention the 8 and 24 pin (cords?) so maybe that's fine.
What I don't read is a cardboard build. No case, just the PSU, motherboard on cardboard for a power up test. I don't need more than that to get a power up.
Just finished putting together my newest build. It is showing no signs of life at all. I will list the components below. I have verified a working outlet, working cables. Made sure power cord is seated into the PSU, Reseated the video card unplugged and replugged the CPU power and Mboard power, tried jumping the board in case the switch is bad. Thought I would throw this out here before I start yanking components. All my other systems are intel so I will not be able to replace the CPU or Mboard with a known good model. I have also pulled everything but the CPU and cooler and disconnected everything but the 8 and 24 pins and have the same result.
MSI B450m Gaming plus
2 x 8 gb Corsair Vengeance LPX DDR 3000 Ram
Asrock Phantom D RX580 8gb
Coolermaster masterwatt 650 PSU
Adata SU 655 SSD
Hopefully this is just a stupid error on my part but having looked this over for the last 30 minutes that is becoming more remote of a possibility. If I cannot figure this out tonight I will start swapping known good GPU, Ram and PSU tomorrow.