There are a number of troubleshooting steps we can follow to resolve this problem, but the uninstall command is
pihole uninstall
If the install is so badly damaged that the pihole command does not operate, then you can run this command from the command line to remove the parts of Pi-hole.
sudo rm -rf /etc/.pihole /etc/pihole /opt/pihole /usr/bin/pihole-FTL /usr/local/bin/pihole /var/www/html/pihole /var/www/html/admin