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Thanks for the notification,
according to the following PayPal page the problem seems to be related to the merchant account registration.
The invalid error means that your billing agreement has been disabled or is inactive.
To enable this , open your developer.paypal.com account, open applications to access your account. click the account you are trying to upgrade to Payment Pro.
Click Profile to find the option Upgrade to Pro in the dialogue box.
please refer to this guide on how to do this https://developer.paypal.com/docs/classic/paypal-payments-pro/integration-guide/WPWebsitePaymentsPro/