Validate Merchant failed for merchantId

Hello,

can anyone help with issue with merchant validation. We receive the next response.

{
  "statusMessage": "Payment Services Exception merchantId=51* unauthorized to service on behalf of merchantId=C6* reason=\"The latter never authorized the former\"",
  "statusCode": "400"
}

We use Sandbox Tester account for testing.

What may be the root cause?
Thank you.