Have you verified that your app isn’t protected with credentials?
This message usually appears when the option “limit access to this app with username and password” is checked (Bubble Editor> Settings> General)
Instead of calling the html2pdf from the page and asking to send the document to the endpoint using the callBackURL, you can do it this way: schedule a Bubble API workflow from your page, in this API workflow generate the PDF, then save the result in your database.