PayPal Pro: receiving a security header error.

I created a registration form last week and needed the payment option at the bottom, once they register.  I created a paypal account and copied my API credentials into the payment add-on on your website.  Once I pasted the html into my website, there was a security error.  

I have never created anything like this or link forms to websites so, I'm doing all this in the dark.

I get an idea and I just go with it.

it gives me an error that says security header not valid.

The form appears to be correctly integrated. However, it could be that the credentials are not valid, or expired, please check with PayPal Pro support. Also, please note that to integrate with PayPal Pro, you need to have a PayPal Pro account.

Let us know if you need more help.