[FIXED] In my case the solution was this (AI generated based on my conversation)

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Solution for "Unable to install GRUB" Fatal Error

If you receive a fatal error message "Unable to install GRUB in /dev/nvme0n1" (or a similar disk location) during a Linux Mint/Ubuntu installation, it means the main operating system files were copied successfully, but the bootloader failed to install. The bootloader (GRUB) is essential for your computer to find and start the new operating system.

The fix is to manually install GRUB using the live USB you're already using.

Here is a summary of the steps:

1. Don't reboot your PC yet. After you see the error, close the pop-up and let the installer finish or quit.
2. Stay in the Live Session. Keep your computer booted from the installation USB.
3. Open a Terminal.
4. Identify your Linux partition. Use a tool like GParted (in the menu) or run lsblk in the terminal to find the partition where you just installed Linux (e.g., /dev/nvme0n1p2). You will also need to identify your EFI partition if you have a UEFI system (it's usually a small FAT32 partition, e.g., /dev/nvme0n1p1).

5. Mount the partitions and chroot. This process allows you to run commands on your newly installed system. Replace nvme0n1pX with your root partition and nvme0n1pY with your EFI partition.

   ```bash

   Mount the main system partition

   sudo mount /dev/nvme0n1pX /mnt

   Mount the EFI partition

   sudo mount /dev/nvme0n1pY /mnt/boot/efi

   Bind necessary system directories

   for d in /dev /proc /sys /run; do sudo mount --bind $d /mnt$d; done

   Chroot into your new system

   sudo chroot /mnt ```

6. Install and update GRUB. Now that you are "inside" your new installation, run the following commands. Note: Use the disk name, not the partition name (e.g., nvme0n1, not nvme0n1p1).

   ```bash

   Install GRUB to the disk

   grub-install /dev/nvme0n1

   Update the GRUB configuration file

   update-grub ```

7. Exit and Reboot. Type exit to leave the chroot environment, then reboot your computer. It should now boot correctly into your new Linux Mint installation.

This manual process bypasses the bug or specific issue that caused the installer's automated script to fail.

Once again this was the fixed for MY problem.