It is known, that for some users the shift-key does not work for displaying the grub menu, but the ESC key should work. Getting the commandline with the ESC key is strange; this should be reached with the c key in the opened grub menu. You should see the grub menu now without interaction.

How do I get to the grub menu in Linux Mint?

When you start Linux Mint, simply press and hold down the Shift key to display the GRUB boot menu at startup. The following boot menu appears in Linux Mint 20. The GRUB boot menu will display with available boot options. Select the one you want and press Enter.

Where is the grub file in Linux Mint?

Re: Where is Grub? If you don’t have a boot “partition” created for it, then it would be in the root partition, the / as seen in lsblk. If you mean the part of grub that points to the /boot, then that’s in the boot sector of the drive. All things go better with Mint.