I'm having exactly the same problem. I installed Pi-Hole on a clean installation of Raspbian. The Pi-Hole installation seems to work correctly - I'm using the Pi address for DNS and it seems to be blocking ads - but I'm unable to get to the admin web interface. Raspbian shows that the device is using the correct static IP. I've tried pi-hole/admin and ip-address/admin with no success. Using ip-address/admin in Chrome returns the following error:
This site can’t be reached
10.0.131.69 refused to connect.
Using pi-hole/admin returns this error:
This site can’t be reached
pi-hole’s server DNS address could not be found.