Seeing this message probably means that the add-on has not been given the necessary permissions to access the contents of your confluence pages. The add-on needs these permissions to be able to transform the page content into a presentation.
To fix this, go to the "User management" settings of your Confluence Cloud instance:
Select the filter to show "System users".
The checkbox "Application access" for Confluence has to be checked. Additionally the add-on needs to be in all of the groups that are necessary to view the pages you want to present:
