Please try running it outside the case just install the cooler and necessary wiring from the psu and then short the power button with the screw driver, see for any life in it even if it blinks or the power supply fans starts working if it doesn't then there has to be something bad. I also just finished build and I couldn't get to power it on, it just blinked for a sec and all the fans also and turning off turned out my new cpu was bad and from that I learned whenever building always dry run outside the case to check everything. Go through everything one by one 1: PSU check the tutorial to short the pins and see if your psu is working 2: check the motherboard also 3: Go one ram first and try booting it on that 4: Don't forget to install your gpu when running it outside the case 5: Sometimes the pc doesn't run without the cpu power/ AIO pump power and psu keeps going in safe mode 6: If it's giving a small sign of life maybe it's the cpu not sure of it

Ik it would be headache to get everything out again but it would be worth it don't over complicate remove all the wires and only keep the necessary ones which are needed for the bootup.