Please help me!

[A_Halfacre]

I just built a PC with my nephew. He saved up money for months and bought all the parts.
Motherboard: ASRock B550M PRO4
CPU: AMD Ryzen 5 5600
Memory: Corsair Vengeance LPX 16GB DDR4 3600
Storage: Crucial P3 Plus 1TB PCIe Gen4 3D NAND NVMe M.2 SSD
Graphics Card: XFX Speedster SWFT309 Radeon RX 6700
Power Supply: Segotep 750W Power Supply, 80 Plus Gold Full Modular PCIe 5.0 & ATX 3.0 Gaming PSU

We are having problems getting it to work. When we hit the power button, it will power on and off every 3 seconds. I think it’s the motherboard but I could use some expert advice???
Here is a video of what it’s doing:

[MEDIA=youtube]f2DpkaPzB6A[/MEDIA]

[phillpower2]

Couple of problems there;

Question first, did you assemble all the parts outside of the case before building inside of the case, this is a must as it determines if the parts all work, what you describe could be a short circuit caused by a standoff in the wrong place, a bared wire or a badly fitted I/O plate.

CPU: AMD Ryzen 5 5600
Memory: Corsair Vengeance LPX 16GB DDR4 3600

The RAM is not appropriate for your CPU, AMD state here up to 3200MHz and if you have XMP or DOCP enabled the RAM will get auto OCd past what the CPU can handle and the PC will fall over.

Power Supply: Segotep 750W Power Supply, 80 Plus Gold Full Modular PCIe 5.0 & ATX 3.0 Gaming PSU

There is one thing that is possibly worse than using a known poor quality brand of PSU and that is using one that is unheard of, for brands of PSU to trust and brands to avoid please refer to the links in my sig.

Graphics Card: XFX Speedster SWFT309 Radeon RX 6700

High end GPUs need high end PSUs from one of the following, Corsair RM range, EVGA, Seasonic or Super Flower, XFX say here a minimum of 650W but recommend 750W, always go with the Recommended.

[phillpower2]

Being that the OP has revisited the forum but chose not to reply this thread will be closed.