Consider the following scenario:
You publish an app to a Microsoft SharePoint app catalog.
An item (such as an Excel worksheet) that uses the app is shared to guest users by its URL.
When you configured sharing, you cleared the Require sign-in check box.
The recipient of the shared item clicks the guest link to open the shared resource.
In this scenario, the user receives the following error message:
App Error We couldn’t connect to the catalog server for this app.
To work around this issue, publish your app to the Office Store. Or, provide an account in the organization through which the user can access the content.
This error occurs because the guest user has access to the content that uses the app, but he or she does not have access to the app catalog.
For more information about apps for Office and SharePoint, go to the Getting Started with Office Development.