Upgraded from 12.0-U8 to 13.0-U6.7. Certbot had worked previously but now any attempts to run it result in this error. As far as I can tell, certbot appears to be a default package (as I don’t recall installing it).
man certbot
Brings up info on TrueNAS-13.0-U6.7 in the shell windows. Should be there by default then. My Core is completely stock. Other than that, I have nothing to add, sorry.