@yakuzakyuu Please do not consider this to be an expert's answer:
When I google your "error message" I read that "N" is for "notice" and may be ignored. Still, it is annoying and I eventually end up here: https://www.digitalocean.com/community/questions/n-missing-signed-by-in-the-sources-list-5-entry-for-http-mirrors-digitalocean-com-ubuntu
Maybe try to see if that fixes it?