Security header is not valid - Error in paypal - WHMCS
This error means one of two things. Either one of the values you've entered for the credentials is simply incorrect, or you're sending sandbox credentials to the live server or vice-versa. Need to double check the endpoint that is being used for the API request to PayPal. It sounds like maybe you're updating the credentials to use sandbox, but the endpoint is still hitting the live server.
PayPal error 10002:Security error Security header is not valid
If your in testing mode you need to modify your initializer and change config.sandbox=true to false. The Railscast is set in live mode.
paypal express checkout =Error: Security header is not valid
generally this error means you are not using the correct API credentials. If you are convinced your credentials are correct then take a look at your endpoint. Sandbox credentials will not be valid in the production environment and live credentials will produce this error in the sandbox
Paypal API error paypal.exceptions.PayPalAPIResponseError: u'Security header is not valid' (Error Code: 10002)
If you are trying to access with live PayPal credentials (not sandbox), you can set:
API_ENVIRONMENT = "PRODUCTION"
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