Cannot start session when headers already sent

PHP Fatal error: Uncaught ErrorException: session_start(): Cannot start session when headers already sent in wp-content/plugins/cleverreach-wc/class-plugin.php:299 Stack trace: #0 [internal function]: {closure}(2, 'session_start()...', 'wp-content/plugins/...', 299, Array) #1 wp-content/plugins/cleverreach-wc/class-plugin.php(299): session_start(Array) #2 wp-includes/class-wp-hook.php(308): CleverReach\WooCommerce\Plugin->start_session('') #3 wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters(NULL, Array) #4 wp-includes/plugin.php(517): WP_Hook->do_action(Array) #5 wp-settings.php(617): do_action('init') #6 in wp-content/plugins/cleverreach-wc/class-plugin.php on line 299