If you are just setting up Optima Express, you may receive this error when viewing the IDX pages. Ensure you have completed the Modify Permalinks step in the Getting Started Guide.
In your WordPress Dashboard, navigate to Settings > Permalinks. Under Common Settings, select Day and name, followed by Save Changes.
Another possibility is that your Site Address and WordPress Address URL's do not match. Navigate to Settings > General, and ensure that both the Site Address and WordPress Address URL fields are the same. Optima Express uses these fields to generate IDX pages.