Recently when whitelisting a number of IP addresses through UFW, I received this wonderful error that left me scratching my head...
ERROR: initcaps [Errno 2] ip6tables: Chain already exists.
Through some research, I have found that UFW tends to freak out if multiple `ufw allow from x` commands are run in quick succession and can end up in a shitty state with duplicate entries. Running the following command should resolve those issues and allow you to add in your entries. I'd recommend you allow 1 second between each command when adding entries to your firewall in the future!
sudo ufw disable sudo iptables -F sudo iptables -X sudo ip6tables -F sudo ip6tables -X sudo ufw enable