GRUB2

grub_is_using_legacy_shim_lock_protocol while booting

Problem; : You are using the old GRUB, probably because an upgrade broke or you didn't run grub-install when needed.

EndeavourOS EXAMPLE, commands need to be modified for your distribution, not modifying these will may your PC un-bootable

Boot in to the Live Installer for the OS (probably what you installed the OS from).

Open gparted from the applications menu and search for your drive in the top right hand corner. The correct drive will have at least 2 paritions, a small EFI partition, and a "File System" with EXT4/BTRFS/Encrypted types.

OPTIONAL : IF you have an encrypted drive (LUKS) you need to "open" the drive, so it can be mounted.
In Gparted an encrypted drive will look like this:

Take the "Partition" value for the LUKS [Encrypted] partition and put it in to this command
sudo cryptsetup open /dev/nvme1n1p2 endeavouros
endeavouros can be anything you like as long as you use the same name in the /dev/mapper line below.

Mount the root (/) partition

sudo mount /dev/nvme1n1p2 /mnt

sudo mount /dev/mapper/endeavouros /mnt

Also mount the boot (/boot/efi) partitions

sudo mount /dev/nvme1n1p2 /mnt/boot/efi

Enter an Arch chroot, this will only exist on Arch based systems.

sudo arch-chroot /mnt

Reinstall GRUB

grub-install

Run

ls -la /mnt/boot/efi/EFI/boot/bootx64.efi

IF you have this file (you see a line containing -rw-r----- 1 root root) then ALSO run the command

grub-install --target=x86_64-efi --efi-directory=/boot/efi --removable

This installs GRUB to the default location your BIOS searches for bootloaders, which is how your PC may be set up instead of using the normal location stored in the EFI memory.

Press Ctrl+D to exit the chroot, then type reboot to restart the system and everything should work.