Internal HDD fails to boot 0n Linux OS

[Taff25]

I have installed Linux Mint 22.1 OS onto the laptop which had been cleared of the Win7 OS manufacturer default. I can run the Linux as a live OS and install it on the HDD but it won't boot from the HDD.  The Boot Repair report suggests as follows:

[code] Suggested repair: ______________________________________________________________

The default repair of the Boot-Repair utility would purge (in order to unsign) and reinstall the grub-efi of
sda3,
using the following options: sda2/boot/efi
Additional repair would be performed: unhide-bootmenu-10s use-standard-efi-file

Blockers in case of suggested repair: __________________________________________

The current session is in BIOS-compatibility mode. Please disable BIOS-compatibility/CSM/Legacy mode in your UEFI firmware, and use this software from a live-CD (or live-USB) that is compatible with UEFI booting mode. For example, use a live-USB of Boot-Repair-Disk-64bit (www.sourceforge.net/p/boot-repair-cd), after making sure your BIOS is set up to boot USB in EFI mode. This will enable this feature.

Final advice in case of suggested repair: ______________________________________

Please do not forget to make your UEFI firmware boot on the Linux Mint 22.1 Xia (22.1) entry (sda2/efi/****/grub****.efi (**** will be updated in the final message) file) !
The boot of your PC is in BIOS-compatibility/CSM/Legacy mode. You may want to retry after changing it to UEFI mode.

[/code]

The problem is that I cannot access the full BIOS  menu. All that can be changed through F10 is the time and date and the Boot Order.  A number of Linux forums have been helpful with advice but they mainly rely on access to the BIOS system.

Any suggestions would be most welcome.  I'm 76 yrs old, so, as you can imagine,  it is very frustrating. However, determination is still strong.