If you can’t add new or delete plugins or do updates in WordPress and you have admin or appropriate privileges then the  DISALLOW_FILE_MODS constant might be causing you problems if it has been set to true.

Open up your wp-config.php (or possibly functions.php) file and look for the following line/code:

define( 'DISALLOW_FILE_MODS', true );

Simply comment it out or change true to false.