Even when you've waited at least 3600 seconds? Which is an hour The command randomly waits between zero and one hour so the Let's Encrypt systems aren't hammered at the same minute every hour.
It is highly unusual for python3 to work, but python not. This looks like an error in your CentOS instance. Does the command whereis python give any result?
This is on a fresh Centos 8 install from the official DVD downloaded yesterday. Then I installed apache, mariadb and nextcloud over it, but i think python3 was already installed by default with the Centos 8 DVD.
Also when you search “Centos 8 dvd python package” you find:
“Python 3.6 is the default Python version.”.
“By default RHEL/CentOS 8 doesn’t have an unversioned system-wide python command to avoid locking the users to a specific version of Python.”
So it seems like “python3 -c” is correct instead on “python -c” on RHEL/Centos 8 right?
Interesting! This is of course not really compliant with the command in the cron job as advertised on certbots page. Perhaps @schoen can be of assistant? Should we update the CentOS cron job on certbots page?