I think Secure Boot require your system to boot with UEFI which requires the disk partition type to be "GPT" instead of "MBR". You can check that in DISKPART then type "list disk" . You must see the asterisk under GPT column of your system (boot) disk. If it's not you can convert it using MBR2GPT utility. After that, you must boot your computer using UEFI (Not UEFI-CSM nor Legacy mode)
Also, do not forget to check Secure Boot mode, in my computer there are 2 mode "Windows" and "Other OS", you must select "Windows".
Also, I think it required TPM at least 1.2 to be enabled, if your system don't have dedicated TPM module, try to check weather you have firmware TPM which is called PTT on Intel system and fTPM on AMD system.
Hope this help . Both of my computer "Surface book 2" and "i7 7700k, 32GB DDR2, NVIDIA RTX3080 PC" don't meet the minimum requirements of Windows 11 because of they equipped with 7th gen Intel CPU (Kaby-Lake) which is stupid as they are both able to run leaked version of Windows 11 perfectly. But from what I've known, the CPU generation is a soft limit , they should prevent the installation of Windows.
Suttisak, I think you have identified the problem: GPT vs. MBR. I had set up all my storage volumes as GPT, but had forgotten to do the same with the OS drive. Yeah, that may be it. Thanks. TPM and all other settings seem to be in order, ready, that is.