The error is pretty clear on this subject. Port 80 is already in use by another thing.
So choose a different port in the Hass.io panel for this add-on (or the add-on using port 80 at this moment).
Have you updated your clients to use pihole as the DNS server? I’ve also found that after I make changes on pihole, I often have to renew my network connection to pick everything up fresh.
Go to your router interface and look for “DNS” (Might be near DHCP). Typically you can set two DNS server IP’s on your router. When clients connect via DHCP it tells the clients to use those DNS servers.
I’ve set my hassio as the main and OpenDNS as backup(If the Pi dies etc), works great for me.