[Koha] Can't enable Plugins

[Sami M. Rasheed]

Dears,Set up PluginsKoha version: 3.16.04.000
I set up the Koha plugin system...
   
   - Create the directory /var/lib/koha/plugins
   - Add the lines <pluginsdir>/var/lib/koha/plugins</pluginsdir> <enable_plugins>1</enable_plugins>" to your koha-conf.xml file
   - Add the line Alias /plugin/ "/var/lib/koha/plugins/" to koha-httpd.conf file
   - Restart your webserver
   - Enable UseKohaPlugins system preference.   

Problem: Plugins still disabled.
 
Plugins disabled!
 To enable Koha plugins, the system preference UseKohaPlugins must be enabled, and the flag enable_plugins must be set in the Koha configuration file
How I can enable Plugins.. Thanks.------------------------------------------------------Sami Al-Asheh