Sorry to hear you are experiencing issues with the plugin. Could you please verify for us what version numbers of WP-Client and WordPress you are currently running?
Also, just to clarify a bit, you are seeing this error message when attempting to view some pages in the frontend while logged-in as a client? Or you are seeing this error while trying to navigate the backend menus of the plugin while logged-in as an admin?
As a diagnostic step, please check your Permalinks settings, and if not already, please change to “Post Name”
See example video here: https://screencast.com/t/oFv7MmcNGb
Even if they are already set to “Post Name”, go ahead and re-save them.
Additionally, can you please verify for us are the WP-Client Theme Link Pages correctly installed and linked in WP-Client–>Settings–>Theme Link Pages? (You should see green checkmarks next to all of the boxes)
See example screenshot here: https://screencast.com/t/lXiOI8Ii
If you continue to experience issues, please feel free to contact us through our official Support Channel at the following link, for a more secure and private support conversation: https://support.webportalhq.com/