Class “PHPMailer” not found

I’m having problem sending SMTP emails and when i check the logs it shows a fatal problem with Unyson plugin.

PHP Fatal error:  Uncaught Error: Class "PHPMailer" not found in /wp-content/plugins/unyson/framework/extensions/mailer/includes/send-methods/class-fw-ext-mailer-send-method-smtp.php:137
Stack trace:
#0 /wp-content/plugins/unyson/framework/extensions/mailer/class-fw-extension-mailer.php(96): FW_Ext_Mailer_Send_Method_SMTP->send(Object(FW_Ext_Mailer_Email), Array, Array)
#1 /wp-content/plugins/unyson/framework/extensions/mailer/includes/option-type-mailer/class-fw-option-type-mailer.php(204): FW_Extension_Mailer->send('xxxx@gmail....', 'Test Subject', '<strong>Test Me...', Array, Array)
#2 /wp-includes/class-wp-hook.php(308): FW_Option_Type_Mailer->_action_ajax_test_connection('')
#3 /wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters('', Array)
#4 /wp-includes/plugin.php(517): WP_Hook->do_action(Array)
#5 /wp-admin/admin-ajax.php(188): do_action('wp_ajax_fw_ext_...')
#6 {main}
  thrown in /wp-content/plugins/unyson/framework/extensions/mailer/includes/send-methods/class-fw-ext-mailer-send-method-smtp.php on line 137