This is a common error that you might get if you are using Paypal Express Checkout or Paypal Website Payments Pro.
This means that there is something wrong with your login information that you are using to authenticate the connection with Paypal. This is similar to the API login and transaction key for Authorize.net. You probably will need to generate a new transaction key. If you generate a new key, please copy and paste the new information, and email it to Center Stage Software, so we can insert it into your settings. Once we have done that, we will let you know, so you can test Webtix and make sure it is working properly.