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.

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 name and put it in to the command sudo cryptsetup open /dev/nvme1n1p2 endeavouros

Open the console app

Mount the root (/) partition

NormalLUKS

mount /dev/nvme1n1p2 /mnt

mount /dev/mapper/endeavouros /mnt

Also mount the boot (/boot/efi) partitions

mount /dev/nvme1n1p2 /mnt/boot/efi

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

arch-chroot /mnt

Reinstall GRUB

grub-install

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