Information
Uncomplicated Firewall (UFW) is a program for managing a netfilter firewall designed to be easy to use.
Uses a command-line interface consisting of a small number of simple commands
Uses iptables for configuration
Rationale:
Running iptables.persistent with ufw.service may lead to conflict and unexpected results.
Solution
Run the following command to remove firewalld
# apt purge ufw
OR
Run the following command to stop and mask firewalld
# systemctl --now mask ufw