Marketpress e-commerce keeps giving me the same message

What do I have to do to get this to work???

I do not want to go to yet another plugin to have the same results.

There was a problem connecting to PayPal to setup your purchase. Please try again.
10002 – Security error – Security header is not valid

Sand Box mode and live mode, just changed my API to see if that would work, nope.

My site is hosted through go daddy and the address is:

https://gagasdolls.com

Please Help:(

https://www.ads-software.com/plugins/wordpress-ecommerce/