Question New computer keeps turning on and off

Nov 25, 2020
Hello! I just built my new pc, here are the components that I am supposed to put in it:
-Aorus z390 I pro wifi (mini itx one)
-2x8gb 3200 G.skill ram
-Corsair sf450 PSU
-Sylverstone rvz03 case
-Gtx 1070ti
Unfortunately, I am facing a lot of trouble with this build, when I first finished it, I tried to boot it without the gpu (with the processor integrated one), it seemed to work well, so I kept building it.
But when I had finished, the computer wouldn't even react when wanting to turn it on, from then, I probably tried everything: booting without gpu, testing psu, trying with my other psu, removing CMOS battery... it then worked fine again when I re-plugged everything and I was mitigated because this didn't make any sense but well, it worked so it was fine, I then unplugged the computer and didn't turn it on for a week because I was on my other computer.
However, when I tried to boot it this morning, another problem has appeared: my pc does start up, but after a few seconds, it turns off then on repeatedly, it seems like the computer turns himself off. This issue did happen, to me earlier and as weird as it seemed, it only happened when I plugged the second system fan, I then proceeded to remove it, then plug it back in, and after a few tries, without me understanding anything, the computer worked fine.
But now, nothing will make it normally start again, I have tried everything again and nothing seems to work, even when the only thing plugged is the motherboard (no ram, no cpu (unplugged the power supply) and no memory) the computer keeps having the same problem of clicking off-and-on repeatedly. I thought about something metallic interfering with the motherboard and causing it to shut down, but even when trying the motherboard out of the case, the same problem remains.
I am thinking of sending back the motherboard back for a new one but I'd like to know what you guys think?