I follow the devstack installation steps:https://docs.openstack.org/devstack/latest/

When I execute this step:./stack.sh I get the below logs in the screen:

 ......

 Installed:
  mod_wsgi.x86_64 0:3.4-12.el7_0                                                

Complete!
+functions-common:yum_install:1341         result=0
+functions-common:yum_install:1343         time_stop yum_install
+functions-common:time_stop:2347           local name
+functions-common:time_stop:2348           local end_time
+functions-common:time_stop:2349           local elpased_time
+functions-common:time_stop:2350           local total
+functions-common:time_stop:2351           local start_time
+functions-common:time_stop:2353           name=yum_install
+functions-common:time_stop:2354           start_time=1499598797
+functions-common:time_stop:2356           [[ -z 1499598797 ]]
++functions-common:time_stop:2359           date +%s
+functions-common:time_stop:2359           end_time=1499598800
+functions-common:time_stop:2360           elapsed_time=3
+functions-common:time_stop:2361           total=95
+functions-common:time_stop:2363           START_TIME[$name]=
+functions-common:time_stop:2364           TOTAL_TIME[$name]=98
+functions-common:yum_install:1352         '[' 0 == 2 ']'
+functions-common:yum_install:1356         return 0
+lib/apache:install_apache_wsgi:67         sudo a2enmod version
sudo: a2enmod: command not found
+lib/apache:install_apache_wsgi:67         true
+lib/keystone:install_keystone:557         is_ssl_enabled_service key
+lib/tls:is_ssl_enabled_service:377        local services=key
+lib/tls:is_ssl_enabled_service:378        local service=
+lib/tls:is_ssl_enabled_service:379        '[' False == False ']'
+lib/tls:is_ssl_enabled_service:380        return 1
+lib/stack:stack_install_service:33        [[ False = True ]]
+./stack.sh:main:798                       configure_keystone
+lib/keystone:configure_keystone:190       sudo install -d -o stack /etc/keystone
+lib/keystone:configure_keystone:192       [[ /etc/keystone != \/\o\p\t\/\s\t\a\c\k\/\k\e\y\s\t\o\n\e\/\e\t\c ]]
+lib/keystone:configure_keystone:193       install -m 600 /opt/stack/keystone/etc/keystone.conf.sample /etc/keystone/keystone.conf
+lib/keystone:configure_keystone:194       cp -p /opt/stack/keystone/etc/policy.json /etc/keystone
cp: cannot stat '/opt/stack/keystone/etc/policy.json': No such file or directory
+lib/keystone:configure_keystone:1         exit_trap
+./stack.sh:exit_trap:474                  local r=1
++./stack.sh:exit_trap:475                  jobs -p
+./stack.sh:exit_trap:475                  jobs=
+./stack.sh:exit_trap:478                  [[ -n '' ]]
+./stack.sh:exit_trap:484                  kill_spinner
+./stack.sh:kill_spinner:370               '[' '!' -z '' ']'
+./stack.sh:exit_trap:486                  [[ 1 -ne 0 ]]
+./stack.sh:exit_trap:487                  echo 'Error on exit'
Error on exit
+./stack.sh:exit_trap:488                  generate-subunit 1499598343 457 fail
+./stack.sh:exit_trap:489                  [[ -z /opt/stack/logs ]]
+./stack.sh:exit_trap:492                  /home/devstack/tools/worlddump.py -d /opt/stack/logs
World dumping... see /opt/stack/logs/worlddump-2017-07-09-111320.txt for details
+./stack.sh:exit_trap:498                  exit 1
[stack@localhost devstack]$ vim  /opt/stack/logs/worlddump-2017-07-09-111320.txt for details

You see there is the error here:

cp: cannot stat '/opt/stack/keystone/etc/policy.json':

And there I tested the below information:

[stack@localhost devstack]$ cd /opt/stack/keystone/etc/
[stack@localhost etc]$ ls
default_catalog.templates  keystone.conf.sample  keystone-paste.ini  logging.conf.sample  policy.v3cloudsample.json  sso_callback_template.html

There is a policy.v3cloudsample.json, no the policy.json.

So, how to do with that when I install the devstack?

My VM system is CentOS 7.2.