So that we are clear on what is happening are you saying that you cannot get a successful transaction regardless of how you test it? If you are getting this error when you try to transact there must be something to it.
To help determine what may be going wrong you should print out the values that you are passing to PayPal on this page you are getting to. You will need to have these values on the page and run a test, then close out the browser or use another one to run another test. On your next test you should compare what you have on this page with what you had during the first test. This will help to clear up what values are being passed to PayPal that may be a problem. If the problem is an invoice number then you should be looking at this value very closely.