"A6" POST code, no boot on Taichi Razer ed. X570

Have a new build that won't boot:

MB: Asrock Taichi Razer Edition X570CPU: AMD 5950XRAM: F4-4000C18D-64GTZR G.Skill (64GB (32GBx2) 4000MHz CL18)

When I turn on the PSU, the board's RGB legs light up (stay on always when there's standby voltage, I assume that's default UEFI setting). When I power it up, and try to enter UEFI, it lets me do it, but shows "A6" on the POST indicator. Shortly after UEFI appears, the PC shuts down on its own. If powered up immediately afterwards, it shuts down shortly after power up (doesn't even reach UEFI\A6 code). If I let it rest for a while, then the initial situation repeats when I reach UEFI but it powers off shortly afterwards.

I checked the manual\googled for the A6 code, it tells me it is as per manual "A6 - SCSI Detect" (very helpful), which people tend to interpret as caused by possible SATA device issues. However, none are attached.

- Storage: none attached for initial testing

- GPU: tested with two different units, no change there

- PSU: same here; all cables are factory supplied (no modded cables used); CPU power connected as per manual

- RAM: set in A2+B2 sockets as per manual; tested with single module vs both, no change; unfortuantely, have no other memory to test the system with

- UEFI: was not the latest version; flashed newest from official site via Flashback, no change

Would be grateful for any pointers you could give.