Cannot enable Ultrafast Boot (Z590 Steel Legend)

I've just finished building a new system, but now I see that I cannot enable Ultrafast Boot in the BIOS.

Motherboard: Z590 Steel Legend 6E WiFi, using the latest v. 1.30 BIOS.
Processor: Intel i9 10900KF
Temporary graphics card: nVidia GTX 660 Ti
RAM: Corsair Vengeance LPX 32GB in 2 modules
SSD: M.2 NVMe 1TB by SK Hynix P31

When I was installing Windows 10 64-bit Pro, I switched "Launch Storage OpROM Policy" setting to "UEFI Only", and storage is in AHCI mode, as listed in this guide, Method 1:
http://forum.asrock.com/forum_posts.asp?TID=1236&title=how-to-install-windows-on-a-pcie-ssd

The CSM option is not disabled in the BIOS, but I don't know if it has any bearing on being unable to activate and keep Ultrafast Boot activated. I did not know that I needed to (do I?) set CSM to Disabled to allow Ultrafast Boot. The guide says "CSM is Enabled by default" and "...simply set the CSM option to Disabled. That has the same affect as setting the Launch Storage OpROM Policy setting to UEFI Only" So effectively CSM should be treated as Disabled in my case.

What is preventing the Ultrafast Boot mode Enabled setting from sticking? Every time I save and exit the BIOS, the Ultrafast Boot setting reverts to Disabled.

P.S.: I did add an HDD for storage, which is a 6TB WD Black model WD6001FZWX. Not sure if it has any bearing on the Ultrafast Boot setting.

Thank you very much for your reply. I suspect my issue is with my graphics card, that does not have a UEFI-compatible vBIOS.

Just to double-check: once I do get a video card that supports UEFI BIOS, can I get ultra-fast boot working with CSM option Enabled (I cannot disable it--it keeps reverting to Enabled) but with "Launch Storage OpROM Policy" set to "UEFI Only"? My Windows install was done with UEFI mode, and all storage is set to AHCI.