I had a similar issue when zoneminder seemed to install ok but the web gui did not load with a 404 webpage not found error.
It looks like the Zoneminder gui was located in a different location that where I expected.
The location that was loading when I went to
http://raspberry-pi/zm was looking for /var/www/zm/ '
The location that Zoneminder installed the web gui files to was /usr/share/zoneminder/ (found this location in /etc/zm/zm.conf under # Path to ZoneMinder web files)
I created a symbolic link for /var/www/zm/ to point to /usr/share/zoneminder/ and it looks like it's worked.
Other possible things to check could be
What's the url you are using to try and access it?
Check firewall. Do you have other web gui's load for other applications? Is port 80 allowed through?
Does the Zoneminder service start ok?
Good Luck
Rick