PC shuts down after 15 or so seconds upon starting up, and then restarts, rinse and repeat

Hey there! It's my first time posting on this sub~ ^^

I put together a new PC build recently combining new parts with some from my old build (which I had for 4 years). I thought I had put it all together correctly, but when I switch it on, it starts up for about 15 seconds, the fans run perfectly fine, but I don't hear any healthy beeping or whatnot, the monitors don't even turn on, and then it shuts down. It then turns back on by itself, only for the exact same thing to happen again, in the same 15 seconds. The PC will endlessly repeat this cycle of shutting down and restarting until I manually force a shutdown.

​

Here are my components:

CPU: Ryzen 5 2600

Motherboard: ASRock B450 Pro4

RAM: Corsair Vengeance LPX 16Gb (1x16)

GPU: GeForce GTX 1660 Super

PSU:  Corsair VS650 650W

HDD: Seagate Barracuda 1TB

Case: CiT Goblin Mesh

​

Everything listed above is brand new, except for the HDD and the case, which I've had for about 4 years.

I also have two optical drives installed, one of which is plugged in and the other is not (it is screwed into the case but not plugged in).

I had checked the compatibility of all these components before purchasing, but due to my own error, I hadn't realized that my old PSU (Antec VP450P 450W) wasn't compatible with my new GPU. So, I bought my new PSU, which is listed above. Note that the issue was the same before and after my PSU change, so I would assume the difference between the two PSUs isn't related to this issue.

After it still failed to work, I went through the usual procedures of checking that cables were plugged in correctly, reapplying thermal paste, etc etc. However, the issue still persists.

I'm kind of a newb when it comes to tech. I know about the components and their roles in general terms, and I know where to plug stuff in when building a PC... but outside of that, I'm fairly clueless. I was going to take my PC to a professional, but since I'm kind of broke right now, I'd like to avoid doing that if I can. The same goes for purchasing new components.

Any ideas would be incredibly appreciated!!

P.S. I wasn't sure whether to put this under troubleshooting or build help, sorry if I messed that up.