I heard that was coming soon but not available yet.
I use cerbot (py) on Linux to generate the certificate and then upload it to my hosting provider. I want one certificate to cover the www and non-www versions of my domain. Currently, only the non-www version is covered. If someone tried to visit my site with the www prefix they get an SSL error. I don’t want to create multiple certificates. Can I make one cert cover my domain both with and without the www?
Yes. Let's Encrypt certificates can include up to 100 names, wildcards or otherwise. You could get a certificate with the names leechristie.com and www.leechristie.com, or leechristie.com and *.leechristie.com.
Certbot has supported wildcard certificates since version 0.22.0.
If you're using cPanel, it's simplest to use cPanel's AutoSSL to automatically issue and maintain certificates, if your host doesn't turn it off.