The reason is that the you have no valid user token associated with your Open eBay Apps application.

A user auth token becomes invalid if either of the following has happened:

• when a user's id or password has been reset by eBay CS for any reason, all the user auth tokens, including the one generated when the eBay user subscribed to your Open eBay Apps application, are revoked ...
• the subscriber in question revoked the user token that was associated with your Open eBay Apps application through My eBay himself/herself

How to get a new auth token?

You need to contact Teapplix customer support and let us know your eBay user name. We need to submit a request to eBay to generate a new auth token for you so you can continue to use the application. The action triggers an updateSubscriberCredentials call that notifies you that a user's Auth token was renewed.