Buildapc

I had this issue, you need to update the bios. However, I couldn't even get it to show up on the monitor, I had to correctly format a USB drive with the update and use the red "flash bios button"

Likely a bios update you need to apply. At one point the B450s weren't going to support Zen 3. AMD reversed course later, but that likely means that a bios update had to come along in order to add the support.

Lots of motherboards, even new ones, still ship with old-ass bios.

Also, remember that memory-training is a thing now -- don't assume that 10 seconds of a black screen means it won't post. Easily wait a minute or more - board could just be in memory-training to get the best speeds.

I just checked for you, the 5700X3D is supported by your mobo with the latest BIOS version. Double check it is the latest BIOS. Reseat the processor, don't overtighten the heatsink, this is a common cause of issues. Remove all RAM, then reinsert 1 stick only. Unplug all USB devices when booting.

I'd say look up the processor compatibility list for the motherboard. It could be that the motherboard doesn't directly support that processor.