Is this an ordinary error to get?

I think the community may be able to help me with this, based on experience;

While trying to send $600 using the plugin, but running in Sandbox with an invalid PayPal API for Advanced, is this a typical error?

Your pledge of $0 is almost complete.

There was an unknown problem processing your payment.
There was a problem connecting with the paypal gateway. (CODE)520003 Authentication failed. API credentials are incorrect.

I am concerned about the plugin displaying $0 for a $600 attempt through the site. I'd like to address this concern now, to minimize surprises when x.com approves the key.