Configure ‘PayPal standard’ keys under Settings > Payment Methods  > PayPal Payments Standard > Settings.

To collect these keys, follow these steps:

1. Login to your PayPal Account, If you don't have one, create a new PayPal account and Login. (Visit https://developer.paypal.com/home/ for login/sign-up).
   
   
2. Once logged in, you need to add email id (used for login/signup) into the corresponding section. Also choose default order statuses for Order Status (initial)/Order Status (pending)/Order Status (processed)/Order Status (completed)/Order Status (others).