This guidance advises on what to do if you get the error 10002 - Security Header is Not Valid when taking payments via PayPal.

Cause & Resolution

The error message means that the API credentials you have entered are not exactly the same as what they have on file for you.  To correct, go to Settings > Card Options > PayPal you should see the API credentials there, compare them to your PayPal ones.  If the problem persists, we suggest you contact PayPal as they would be in the best position to identify the cause of the error.