Hello, Check the URL which is shown in the address bar of your browser when attempting to login as the buyer and make the payment. If this doesn't start with "https://www.sandbox.paypal.com" then you won't be able to use a sandbox testing account to make the payment. If the URL starts with "https://www.paypal.com", then you have reached our production site and only a production account can login and confirm / make the payment. Thanks !
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