This thread (and a few others with the same error message) might help:
Use lsblk -f to determine your btrfs system partition and substitute that in place of /dev/nvme0n1p2 in this tutorial, which is more structured than the post you linked.
This applies also to determining your ESP later on.
By-the-way, you could also try the tools provided by the live USB, instead of that manual procedure.