Had the same problem, here’s how I solved it.
First off, /usr/local/CyberCP/lib/python3.6 was not present on my system, but python3.8 instead. So I created a symbolic link for force the path to traverse python3.8 instead (commands issued as root, otherwise prepend sudo ):
$ ln -s python3.8 /usr/local/CyberCP/lib/python3.6
Second, directory /usr/local/CyberCP/lib/python3.6/site-packages/tldextract/.suffix_cache and everything beyond belonged to root:root. So I changed ownership of directory and beyond to cyberpanel:cyberpanel:
$ chown -R cyberpanel:cyberpanel /usr/local/CyberCP/lib/python3.6/site-packages/tldextract/.suffix_cache
And then I was able to issue DKIM certificates!
@simyseng : Can you confirm the ownership of .suffix_cache and below on your server where it ran correctly?
@CyberPanel: please enable MarkDown in forums!