PayPal gateway has rejected request. Security header is not valid (#10002: Security error). how to resolve it
Credits : Vimalnath for link
The Security header is not 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 accidently a space added, this would trigger this error. Doublecheck this settings in the SDK or in the admin panel of your third party shopping cart.
Wrong Endpoint This error would come up if you send the data to the wrong endpoint. Make sure that you sending 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 in test or live mode, regarding which credentials you are using.
You can check for your credentials here too:
Just Substitute the user, pwd and signature and enter in your browser. You should get ACK = SUCCESS if you have input your credentials correctly.
You can also get your credentials here : https://www.paypal.com/cgi-bin/webscr?cmd=_get-api-signature&generic-flow=true