"Legacy Support" won't disable in BIOS setup

[HSILA]

Hi everybody
I had Win10 Enterprise running on my laptop , with MBR Partiotion type. Then i decided to change it to GPT, so i backed up everything and wrote windows iso on a flash memory using RUFUS (UEFI mode).
I tried to install the windows (I cleaned the hard disk and converted it to gpt using DISKPART during Windows install), everything was fine until the first restart. I got this error :
"No bootable device -- insert boot disk and press any key."
Then i thought i should change something in BIOS setup , so i headed there and tried to disable "Legacy support" , but after each restart it been set to "enable" again, rejecting the change.
I checked the BIOS version , it was F.36 and the latest version was F.57. So i decided to update the bios. I reverted to MBR and tried that but the update installing was failed , even i tried updating  through usb but it didn't work neither.

what should i do to install a copy of windows with gpt partitions?

[CherylG]

Did you also re-enable Secure Boot in BIOS?

[HSILA]

I tried that too, but the secure boot option was gray and unselectable, Even after disabling "legacy support".

[CherylG]

Under Boot options is it set to UEFI?

Look for the setting to Reset to Default settings in BIOS.Save and Exit.

[HSILA]

The BIOS setup looks like this :

After selecting "Boot Options" :

Secure boot option is grayed out , and i cannot disable "legacy support" (as I explained). So it seems that my laptop supports the UEFI but I cant use it to install Windows. And I also tried "setup defaults" but it didn't work neither.

[CherylG]

Highlight "Clear all secure boot keys" and tap enter key.

Highlight "Load HP factory default keys" and tap enter key.

Tap F9 key to load BIOS Default settings. Save and Exit. On reboot check BIOS again to see if you have Secure Boot enabled or if you can now enable it which should disable Legacy boot.

[HSILA]

At the first , the Clear All Secure Boot Keys option is unelectable. It will be selectable if i set legacy support to disabled. Then i can clear secure boot keys and load default keys. Then secure boot option will be black and selectable. I set that to "enabled" . But after each save and exit (F10) , changes will revert to default (Legacy enabled and secure boot disabled).

I can use F9 (Boot Device Options) at the first to find image boot loader and install UEFI Windows but as long as legacy support option is unchangeable, system will not recognize the UEFI Windows and each time i should load the bootloader manually .