If you are trying to use Magisk to get root in Lineage, you don't need to do that dance with patching the bootloader. Just use adb sideload to flash the Magisk zip from HERE and you will have root, no muss no fuss. (I'm still using an old one, 21.0) I never even open Magisk Manager, although the zip installs it.
I'm sorry, I don't know what's going on with fastboot. I assume you are booted into the bootloader, not recovery, when you are doing that (I'm not trying to be insulting by pointing to the obvious-- when I get frantic I can miss incredibly obvious things and am happy when people