Apple Pay on the Web - Merchant Validation fails with 400 and 403 status returned by Apple

My company has implemented Apple Pay on the Web. We are getting 400 and 403 status codes returned from Apple during the merchant validation process and have no idea why. All the required fields described here are provided. (https://developer.apple.com/documentation/apple_pay_on_the_web/apple_pay_js_api/requesting_an_apple_pay_payment_session)

We completed the configuration of environments, registering and verifying domains, and have even tested successfully on lower environments.

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Update from OP:
So here is the actual response from Apple during Merchant Validation:
{
"statusMessage": "Payment Services Exception merchantId=9..9 unauthorized to process transactions on behalf of merchantId=9..9 reason=\"9..9 never authorized mass enablement transactions to occur via 9..9\"",
"statusCode": "400"
}

How do we get authorized for mass enablement?

Hello, how did you solve your problem