CentOS 6 is end-of-life and doesn't get any updates any longer since November 2020. You might search the CentOS package manager for certbot, but even if it's in there, you might find yourself with a very ancient version of certbot.
If upgrading CentOS isn't an option (although I recommend it VERY much! Running end-of-life systems is never a good idea) and you can't install certbot on CentOS 6, you might want to look to other ACME clients.