If you get error code 10002 with the message 10002: Security Header Invalid, you have incorrect API credentials. Check that the account that will receive payments was used to generate the API credentials. If you're using PayPal's Sandbox environment, you must generate API credentials from within the Sandbox business account rather than a Live account.

https://developer.paypal.com/docs/classic/lifecycle/sb_credentials/

https://developer.paypal.com/docs/classic/api/apiCredentials/#create-an-api-signature

Error 10501 This transaction cannot be processed due to an invalid merchant configuration. = you need to to enable Payments Pro in your sandbox account

https://developer.paypal.com/developer/accounts

• 1.Login to your developer.paypal.com account
• 2.Go to Applications -> Sandbox Accounts
• 3.Click the sandbox account you want to upgrade to Pro, and then click 'profile' tab
• 4.There should be an Upgrade to Pro option in the dialog that pops up