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 request. Security header is not valid (#10002: Security error)”. The Security header is not a valid error is only caused for two reasons:
Wrong credentials 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.