Custom Build Computer Shuts off Immediately After Turning On

"Unfortunately motherboards these days do not come with a CMOS speaker, so a motherboard speaker would need to be plugged in to hear the sound.

Did you already try the old two sticks of ram to see if it would post?

Try doing a CMOS reset which should be explained how to in your motherboard manual."

i thought the z370 is only compatible with 8th gen Intel CPU's? if it was a z270, they're compatible with 7th gen Intel CPU's. You have a 7th gen Intel CPU (i5-7500) so the new motherboards aren't backwards compatible, everything else will work, you will just need to get a new CPU =), if anything you can use http:www.pcpartpicker.com to help find compatibility in the future.