Following are checklist you should verify, probably following is the basic reasons for an error.
1: Be sure the merchant account has Pro permissions. Also, be sure the
app has gone through the developer-approval process.
2: Also, be sure the app has been approved. The app has basic
permissions until you submit it for approval.
3: You may have a Business account but to accept credit card payments
in the live environment your account must be approved for Paypal.
4: You should contact PayPal Developer Support. File a ticket at
PayPal merchant technical Support. Click on the Contact Technical
Support link to file a ticket.
5: You can call to PayPal Customer Care, They will verify your name,
DOB, Verification Code (Code will be shown in your PayPal Dashboard in
Customer support page) etc, they will fix your issue in a minute or
they will help you to fix your issue.