Alienware Aurora R4 power button issues (?)

So, today, I got an Alienware Aurora R4 from a dude on the buying/selling app Offerup.

I plugged in the desktop and proceeded to press the power button but there's nothing that happens. I'm not sure why it doesn't work but it's quite disappointing.

However I can turn on the desktop if I hold the black button (I believe its the diagnostic button) and the green light turns on and the desktop will proceed to boot up and open the windows home screen. As soon as I take my finger off of the button it turns off...I'm not sure how to fix this, perhaps there's not power going to the power button?

I opened the side of the desktop and I seen an orangish/amber colored light that read "power". If its receiving power, how come it wont work when I press the power button? I need help guys!