Build Advice New build not working

Jun 20, 2019
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Yesterday I build a new system
Mb: Asus Prime H370-Plus
CPU: Intel 8400
Ram: Curcial 2666mhz 16gb x2
and there's no graphics card currently.

After assembling everything and connecting it to the power the LEDs on the MB turned on, tried pressing the start button but nothing happened, tried booting it up using a screwdriver, nothing happened.
However, it turned on when i removed the 8 pin CPU power, and the fans started turning

So what might be the problem?? I'm totally confused.
All the parts are brand new.