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

Open the console app
OPTIONAL : if you have an encrypted drive you need to "open" the drive, so it can be mounted.
In Gparted the correct drive will look like this:

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

Mount the root (/) partition

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.