Error in theme

I have an error on my website. Can you help me with that?

PHP Fatal error: Cannot redeclare sydney_set_import_files() (previously declared in /domains/marloesjanknegt.nl/public_html/wp-content/plugins/sydney-toolbox/demo-content/setup.php:18) in /domains/marloesjanknegt.nl/public_html/wp-content/themes/sydney/demo-content/setup.php on line 21, referer: https://marloesjanknegt.nl/wp-admin/admin.php?page=seed_csp4

The page I need help with: [log in to see the link]