That typically means the account settings are incorrect. I recommend the below.
PayPal Business (email or secure merchant ID): Set this to your Merchant ID. You can find this on the below page under "Account options".
Code:
https://www.paypal.com/myaccount/profile/
PayPal Main receiver account (email) (if Business is different from your main account): On that same above page this should be set to whatever email address is marked "primary".
Your PayPal Identity Token: This can be found on the below page under "Payment data transfer"
Code:
https://www.paypal.com/businessmanage/preferences/website
Ensure there's no copy-paste errors with the identity token. For example if you just double click it to highlight it it won't select the full string.