A quick search of git@github.com: Permission denied (publickey). reveals...
GitHub isn't able to authenticate you. So, either you aren't setup with an SSH key, because you haven't set one up on your machine, or your key isn't associated with your GitHub account.
You can also use the HTTPS URL instead of the SSH/git URL to avoid having to deal with SSH keys. This is GitHub's recommended method.
The error is occurring due to ERR! /usr/bin/git ls-remote -h -t ssh://git@github.com/anerdins/nibepi.git
It has nothing to do with it - it's because you chose to update buffer-perser.
You will find any npm node you attempt to update, this will happen.
The point is GitHub have changed the way you clone repos. This other node you mention has a dependency to a GitHub resource. It's failing to access it.
im guessing, because you were experimenting with newer versions of nodejs / npm
at some point your package-lock.json got updated to a newer version.
when you reverted back to nodejs 12 the lock file stayed with v2 causing that error message.
i dont know much about npm but if it was me i would take a chance **
and rename the package-lock.json to package-lock.old and
run npm install from .node-red folder to re-install and recreate it with a correct version.