I had the same problem. Used stable/ocata OpenStack version on Ubuntu 16.04. My issues was that Devstack was on master. You also have to clone Devstack as stable/ocata, not only OpenStack services.

I run ./stack.sh from master: git branch *master . When I checkout stable/ocata fro all service (/opt/stack... devstack, keystone, glance, horizon, cinder and other) I solved this problem.

For what it's worth, I too have this problem. None of the suggestions help. Near the end of running 'stack.sh' I am blessed with this gibberish here:

+./stack.sh:main:1059                      create_keystone_accounts
+lib/keystone:create_keystone_accounts:383  local admin_project
++lib/keystone:create_keystone_accounts:384  openstack project show admin -f value -c id
Failed to discover available identity versions when contacting http://192.168.1.100/identity_admin. Attempting to parse version from URL.
Could not determine a suitable URL for the plugin
+lib/keystone:create_keystone_accounts:384  admin_project=
+lib/keystone:create_keystone_accounts:1   exit_trap
+./stack.sh:exit_trap:487                  local r=1
++./stack.sh:exit_trap:488                  jobs -p
+./stack.sh:exit_trap:488                  jobs=
+./stack.sh:exit_trap:491                  [[ -n '' ]]
+./stack.sh:exit_trap:497                  kill_spinner
+./stack.sh:kill_spinner:383               '[' '!' -z '' ']'
+./stack.sh:exit_trap:499                  [[ 1 -ne 0 ]]
+./stack.sh:exit_trap:500                  echo 'Error on exit'
Error on exit
+./stack.sh:exit_trap:501                  generate-subunit 1490374530 163 fail
+./stack.sh:exit_trap:502                  [[ -z /opt/stack/logs ]]
+./stack.sh:exit_trap:505                  /home/stack/devstack/tools/worlddump.py -d /opt/stack/logs
World dumping... see /opt/stack/logs/worlddump-2017-03-24-165813.txt for details
df: '/run/user/1000/gvfs': Permission denied
+./stack.sh:exit_trap:511                  exit 1

check if apache2 is running. Generally happens if any server is running on port 80

I was receiving this error when running "$ openstack project create --domain default \". Turns out that httpd.service was not running on my computer and causing the error.