Welcome to Tom's Hardware my friend! I think you will find this forum to be alot of help for any future issues or questions you may have
I wish I had enough space in this thread to put all the links to forums and sites that explain how unwise it is to buy a pre-build from basically any retailer. no big deal, I don't blame you or anything, my first PC was a prebuild as well, we just gotta learn the hard way sometimes. but we're passed that so lets just get to the issue.
It is very helpful when presenting a problem to also list your PC's Specifications like so:
CPU Cooler: (If not using stock fan)
And sometimes the Case, if it's an odd one or the issue is thermal
If you don't know how to find your specs, just let me know and ill show you. it's quite easy
NOTE: When using a single stick of RAM in a your ASRock Pro4 motherboard, it is essential that it is placed in slot A2, (Second slot from the CPU), not the first slot as shown in your video
Edit: I see the video and it looks like you have a Ryzen Processor, but besides the motherboard, there isn't much we know about your build and it's parts yet.
It will help to post your system components.
Is that an APU...I don't see a graphics card in the system.
To rule out the front panel on the PC case, disconnect all cables from the case front panel and short the power header on the motherboard with the tip of a screwdriver.
The power switch header is usually labeled “PWRSW,” “PWR_SW,” “PWR_BTN,” PWRBTN
Its possible the power button on your case is improperly plugged it. view your manual again > http://asrock.pc.cdn.bitgravity.com/Manual/B450 Pro4.pdf
and go to page 28, there it will show the layout of the F_PANEL plugin. try using a screwdriver or paperclip to short the "PWRBTN# " and "GND" pins INSTEAD of using the case power button. first mage sure all power is plugged into the Motherboard, EPS (CPU), and HDD SATA, make sure your PSU is plugged in and switched on, then short those 2 pins to simulate the pressing of the power button
Edit: To find where the F_PANEL plug in, view Page 7 on the manual and see point "16" on the bottom right of the board