Hello,
Zetadocs Express can be used with SharePoint on-premise and SharePoint online. More details available here.
In the screenshot provided I cannot see the SharePoint URL that is specified on the Zetadocs General Settings page, are you including the name of the document library (ie 'Zetadocs') in the path? If yes, then please remove it and check if you get the same issue.
- Has the 'Zetadocs Content Type' and 'Zetadocs Document Library' features been activated OK? (Go to 'Site Settings | Manage Site features').
- Is the Zetadocs Content type available to the Zetadocs Document library? (Go to Document Library Settings).
Thank you.