new builds normally fail to post due to user error.
or, even bad configuration prior. (i'll explain in a minute.)
my dad rebuilt a computer once, and brought it to me bacause he couldn't get it to display... he forgot to plug in the PCIe power cables to the GPU.
i've had friends bring me machines that they failed to secure the RAM in all the way. or forgot to plug in the 8pin CPU power.
recently i swapped my PCIe-3.0 GPU out for a PCIe-4.0 GPU. now normally this doesn't matter, but i use a riser cable in my mini-ITX build. the riser is only rated for PCIe-3.0.
I had to put my old GPU back in, then go into my BIOS settings to switch the PCIe slot to force 3.0, otherwise the machine fails post on a VGA error code with the new GPU.