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We also tried using PayPal Pro as the payment gateway and also got the Security Header is Not Valid Error.Asked by ACFPA on March 08, 2016 at 08:57 PMWe also tried using PayPal Pro as the payment gateway and also got the Security Header is Not Valid Error.
This happens when you have entered incorrect credentials for your PayPal Pro account during the integration process. You must've copied the data from a source and pasted it into the PayPal integration window. Please be reminded that spaces (" ") might also be copied by doing that.
I would suggest you try to have your credentials to a notepad or any text editor so you will be able to easily determine if there are any spaces included. Meaning, the error you're receiving is caused by entering incorrect PayPal API Username, Password, and/or Signature.