As usual, logging this so I don’t lose track of it later: https://www.linode.com/community/questions/18963/certbot-error-problem-binding-to-port-80

And the original content in case it goes missing:

This is a common CertBot error when using the Standalone plugin (--standalone flag) during the certificate renewal/creation process. In this case, CertBot will attempt to use port 80 to create a temporary web server. You have a few options moving forward.

Temporarily stop your existing web server

To continue using the --standalone plugin, your existing web server must temporarily be stopped. The exact command to do this varies based on your operating system and web server software. Here’s common commands to stop Nginx and Apache, respectively:

Ubuntu and Apache: sudo systemctl stop apache2
CentOS 7 and Apache: sudo systemctl stop httpd 
Ubuntu or CentOS 7 and Nginx: sudo systemctl stop nginx