Reinstall Grub2

custom.txt

if [ "${grub_platform}" == "efi" ]; then
	menuentry "Google" {
		insmod part_gpt
		insmod fat
		insmod chain
		set root='(hd0,gpt12)'
        	if [ x$feature_platform_search_hint = xy ]; then
			search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt12 --hint-efi=hd0,gpt12 --hint-baremetal=ahci0,gpt12 7602-3D50
		else
	  		search --no-floppy --fs-uuid --set=root 7602-3D50
		fi
		chainloader /EFI/Boot/bootx64.efi
	}
fi

default.txt

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_TIMEOUT=5
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX=""

# Uncomment to enable BadRAM filtering, modify to suit your needs
# This works with Linux (no patch required) and with any kernel that obtains
# the memory map information from GRUB (GNU Mach, kernel of FreeBSD ...)
#GRUB_BADRAM="0x01234567,0xfefefefe,0x89abcdef,0xefefefef"

# Uncomment to disable graphical terminal (grub-pc only)
#GRUB_TERMINAL=console

# The resolution used on graphical terminal
# note that you can use only modes which your graphic card supports via VBE
# you can see them in real GRUB with the command `vbeinfo'
#GRUB_GFXMODE=640x480

# Uncomment if you don't want GRUB to pass "root=UUID=xxx" parameter to Linux
#GRUB_DISABLE_LINUX_UUID=true

# Uncomment to disable generation of recovery mode menu entries
GRUB_DISABLE_RECOVERY="true"

# Uncomment to get a beep at grub start
#GRUB_INIT_TUNE="480 440 1"

GRUB_DISABLE_OS_PROBER="true"

Install Grub.txt

sudo mount -t btrfs /dev/sdb /mnt/disk
cd /mnt/disk
sudo mount --bind /dev dev
sudo mount -t proc proc proc
sudo mount -t sysfs sysfs sys
sudo chroot ./

ls /dev/disk/by-uuid -alh

nano /etc/grub.d/40_custom
nano /etc/fstab
update-initramfs -u
update-grub
grub-install --force /dev/sdb

exit
sudo umount dev
sudo umount proc
sudo umount sys
cd ..
sudo umount /mnt/disk

Repair Grub.txt

sudo apt update
sudo apt install efibootmgr -y
efibootmgr -v
sudo mkdir /mnt/disk
sudo blkid
sudo blkid

sudo mount -t ext4 /dev/nvme0n1p6 /mnt/disk
sudo mount -t vfat /dev/nvme0n1p1 /mnt/disk/boot/efi
cd /mnt/disk

sudo mount --bind /dev dev
sudo mount --bind /dev/pts dev/pts
sudo mount -t proc proc proc
sudo mount -t sysfs sysfs sys
sudo mount --bind /sys/firmware/efi/efivars sys/firmware/efi/efivars
sudo chroot ./

ls /dev/disk/by-uuid -alh

nano /etc/grub.d/40_custom
nano /etc/fstab
update-initramfs -u
#grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=debian
grub-install --target=x86_64-efi /dev/nvme0n1
grub-install --recheck /dev/nvme0n1
update-grub2
exit

sudo umount dev/pts
sudo umount dev
sudo umount proc
sudo umount sys/firmware/efi/efivars
sudo umount sys
cd ..
sudo umount /mnt/disk/boot/efi
sudo umount /mnt/disk

efibootmgr -v

1. Boot into Windows 10.
2. Press the Windows + R keys to open the Run utility.
3. Type cmd and press Ctrl + Shift + Enter to open an elevated Command Prompt.
4. Now, type the following command and press Enter:
   bcdedit /set {bootmgr} path \EFI\debian\shimx64.efi
5. After you run the above command, it will change the bootloader back to Grub.
6. Restart your PC and you should be greeted with the Grub Menu.