Please re-open the App from SharePoint
This error message appears in the following circumstances:
If you browse directly to the SharePoint Online Connector IIS Site - The full error message is Could not create context. Please re-open the App from SharePoint. To view and update configuration settings for the SharePoint Online connector, you must go to a SharePoint Site’s contents, as a Site Admin, and click the connector app. Most of the information/settings is not site-specific, however the few pieces of information that are require access the connector configuration page from the SharePoint site itself.
Incorrect app principal configuration - ensure the App domain, and Redirect URL are correct.