What is the exact URL you are using to access Pi-hole's web interface?
Pi-hole's supported web server is lighttpd.
You are using nginx instead:
*** [ DIAGNOSING ]: Ports in use
[✗] tcp:0.0.0.0:80 is in use by nginx (https://docs.pi-hole.net/main/prerequisites/#ports)
[✗] tcp:[::]:80 is in use by nginx (https://docs.pi-hole.net/main/prerequisites/#ports)
To have those webservers coexist, you could try to move lighttpd to different port.
For a Docker Pi-hole, set your container's WEB_PORT environment variable.
For a bare-metal Pi-hole, you can overwrite the port in your /etc/lighttpd/external.conf.