paypal setup errors

i have been setting up paypal as one of three payment gateways, i have set up paypal with the right sandbox codes, but do not yet see the site respond to test purchases in test mode, instead i get a 404 page . if anyone has any insight, they would be welcome here, Thanks

https://cybrgigs.com/service-payment-2/Received%20error:%20401Raw%20response:%7B%22error%22:%22invalid_client%22,%22error_description%22:%22Client%20Authentication%20failed

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