had that issue also, and that issue is also known on their forums.
pi-hole install seems ok.
but only thing that was missing was that password for admin page was not set.
maybe install script wanted to end with pihole -a -p to set password of admin page and somehow could not find executable
i executed:
echo "alias pihole='/usr/local/bin/pihole'" >> ~/.bash_aliases && source ~/.bash_aliases
and after that pihole -a -p to set password