Wordpress install: The 'mysqli' extension was not found

• OS: Ubuntu 14.04.4 LTS
• Version de Plesk:12.0.18 Mise à jour n° 92

Hi, please help me, i am encountring a problem when i try to install wordpress , this is the error :
The 'mysqli' extension was not found.
any suggestion please ? thank you

* Module fcgid already enabled
* PHP 5.5.9-1ubuntu4.16 (cli) (built: Apr 20 2016 14:31:27)