I am having a problem but with express:store on expressionengine and paypal_express. I have inputed all the details but I was receiving a 'Security header is not valid" I deleted the payment method and re-created it but I am now getting another error 'returnURL is invalid' I have remove the .htaccess thinking this may interfere with it but I am at the end of my teather here. Any help or advice would be greatly appreciated.
When you see the PayPal error:
Security header is not valid
This generally means you have PayPal set to "Test Mode", but you are using a live account, or vice versa. PayPal test mode is only for when you have a PayPal developer account (which is a completely separate login/password, you cannot use it with a regular account).