Your PHP installation appears to be missing the MySQL extension which is required by WordPress.

sudo apt-get install php7.2-mysql

Meaning your WordPress installation is most likely outdated. 
Since the 'MySQL' extension is deprecated (no longer supported) 
from PHP 7.0 and beyond, changing your PHP version to 5.6 or lower
will fix the issue. This can be done via your website's 
control panel's PHP Configuration section.