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Security header is not valid
This is a PayPal API error which signifies your API credentials are invalid, check to make sure that you have the correct credentials for the correct mode. PayPal doesn't allow you to use your live credentials within sandbox mode. If this doesn't help solve the error you will need to contact PayPal, we've seen 1/100 times an account needs to be manually approved by PayPal for API access first.
Invalid Configuration: This transaction cannot be processed due to an invalid merchant configuration. (Error Code: 10501)
This error is the result of an issue with your PayPal account. Please contact PayPal support about this to get it resolved.
Before you do that though, you should confirm your configuration, making sure you're using the appropriate credentials for the proper method (e.g. live/staging).