Password Plugin with SQLite3 on ubuntu 16.06

[rdas]

Hi,
I am trying to configure the password plugin. The database is sqlite. The path to the sqlite file is (write permission given to www-data group):

$config['password_db_dsn'] = 'sqlite:////etc/postfix/sqlite/db/postfix.db';

I am unable to connect to the database. I see the following message in logs/errors:

DB Error: [14] unable to open database file (SQL Query: update...

I cannot find out why this is happening. Do I need to install any additional pear package?

Thanks.