Another option that you could try by checking the wp-config.php
file and make sure if you have the codes something like this.
define('DISALLOW_FILE_MODS', true);
Through this code you are not allowed to add any theme / plugin and even not able to edit the codes directly from your site. You can disable this setting by just modifying the DISALLOW_FILE_MODS to false.
define('DISALLOW_FILE_MODS', false);