I'm receiving "error transaction" in PayPal pro

Ao submeter o formulário, com conta de testes do sandbox, recebo a seguinte mensagem: "Security header is not valid"... 

Agradeço a ajuda.

Please note that the error related to 'Security header' shall be displayed when the API credentials are incorrect. Please make sure that you have copied and pasted the credentials correctly without any leading or trailing spaces in the payment integration and try again.

Please check and let us know if you are still experiencing any issues with the form. We will be happy to take a look again.

Guide: How-to-Integrate-Form-with-Paypal-Pro