Please provide more information on the type of transaction you're attempting, the specific information about the details of the transaction request and how you're attempting to make the payment (via PayPal account, via debit/credit card without an account ect..)
Also provide the merchant_ids for both the merchant account and the buyer account. Instructions on how to retrieve the merchant id for an account can be found in the post "pinned" to the top of this forum, named "how to report a sandbox issue".
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