A few, just a few machines require a current BIOS. I see updates from late 2016 at https://support.hp.com/us-en/drivers/selfservice/hp-z420-workstation/5225033

Another common problem is of course drivers. But a proper PC with NO DRIVERS will default to some basic video mode to let us install the drivers.

Remember for W10 we always install in the order of motherboard, audio, video, then LAN, WLAN etc.

Another fine test is a boot to SAFE MODE which is documented on the web. Try that and get back to me.