One of possible reason is you/automated installation might have turned off installation of new themes and plugins in wp-config.php.
Make sure you do not have DISALLOW_FILE_MODS true in your wp-config.php file.
define( 'DISALLOW_FILE_MODS', true );
If it is then remove this line. Enabling this option in config file will disallow updates and installations of new themes and plugins from WordPress admin panel for security reasons.
You can read more about DISALLOW_FILE_MODS on codex.