Are you following the guide and chrooting into the existing install while using a live USB?
It sounds you have failed to do so and instead are in the grub rescue shell.
Which is an entirely different scenario … a quick search guide is here:
But while that is a thing - I rather doubt it will be helpful to you.
According to other posts here
(ex: Symbol `grub_is_lockdown` not found - #6 by wils0n)
You will need to reinstall grub, which cannot be had from the rescue shell.
According to other posts
(ex: [Testing Update] 2021-06-08 - Kernels, Cinnamon 5.0, Gnome 40.2, Cutefish 0.2, Deepin, Firefox - #52 by Termy)
This is simply the wrong path used for the ESP.
(in that case also fixed by reinstalling grub using the correct path)