Vostro 470, cannot enable secure boot

I am having difficulties enabling the SECURE BOOT option in the BIOS of my Vostro V470 desktop.

I can switch between UEFI and Legacy, but I I cannot enable the SECURE BOOT option.  I know that I must be in UEFI mode and that works, but even if I press F12 during boot, and select the switch to SECURE BOOT option, it won't stick and won't boot.

Similarly, with F12, it shows OPROM is enabled, but I cannot disable OPROM.

I updated to the latest A14 BIOS.  I even downgraded to A13 and tried it without luck.  Then I reflashed to A14 but same results.  I can set UEFI, but the SECURE BOOT and OPROM option are greyed out.  I can move the cursor to those option, but I cannot change them and remain greyed out.

Things I have tried that didn't help:

reset BIOS settings to factory and saved changes.

I tried brand new SSD drives, ran diskpart to wipe drives. (maybe I will try a spinning older drive next)

I tried with and without RAID, still cannot enable SECURE BOOT and cannot disable OPROM.

I pulled the plug and removed the battery to clear the CMOS and waited an hour and reinstalled battery.

I reinstalled Windows 10-1903 many times (64 bit) and used different media, still cannot get secure to work.

Strange as I seem to remember when I was setting up the PC to sell to a client last month, that it was configured as SECURE BOOT but I switched to LEGACY as the client wanted Windows 7.  They canceled the order and now I want to use this PC for myself with Windows 10 with SECURE BOOT.  It is currently in UEFI/RAID with Win10Pro-1903 and everything is working perfectly except there is no SECURE BOOT and OPROM is enabled.

I am running out of options.  Do I just give up and use it without SECURE BOOT?

I am worried that this might be a symptom of something else that is wrong.

Thanks

Richard