If you use certbot-auto to install the client, part of the installation process involves compiling some crypto library (IIRC) which tends to fail on low-memory systems, hence the need for swap.

Packaged versions of certbot (from your distribution's package manager) don't suffer from that problem and have fairly low memory requirements.

Yeah right swap memory. I'm a server management newbie. This was a cent os instance and apparently it didn't have a native certbot so I had to use certbot auto.