When hooking up payment and shipping in Magento there are a few common errors that you may see. The most common one is one from Paypal that says “PayPal gateway has rejected the request. Security header is not valid (#10002: Security error)”.
Why Security Header Is Not Valid:
The Security header is not a valid error is only caused for two reasons:
Make sure that you’ve put your API Username, API Password, and API Signature correctly. Sometimes it happens that during copy and paste there is accidentally a space added, this would trigger this error. Double-check this setting admin panel of your Magento store. Worst comes to worst, type out all the information by hand instead of copy, pasting it.
The Wrong Endpoint
This error would come up if you send the data to the wrong endpoint. Make sure that you send the live credentials and data to our live endpoint. When you want to test your store make sure that you use our test endpoint and the credentials from your sandbox test account. If you are using a third-party shopping cart, make sure that your store is running a test or live mode, regarding which credentials you are using.