PayPal payment error message: Security header is not valid

I'm getting an error message when trying to pay for this workshop. How do I fix this? 

I did a test submission on your form and I was able to replicate the issue. Here's a screenshot of the error message:

The issue here is that the API credentials you entered is incorrect or invalid. Please re-enter the credentials correctly and make sure there are no extra spaces or extra characters. You try pasting it first in a text editor program like Notepad to ensure that information is correct.

Hope that helps. Let us know if you're still having issues. Thank you.