On my PORTAL'S home page I have featured contents that shows web apps and web maps that have been shared with a group called featured apps and maps. When I add a single display operations view app that can be opened in a browser to this group so I can select to open it on the home page, I get a web page not found error, but web apps and web maps open fine from the home page. Can you not open operation views from the home page in portal?
When you click on an operation view listed under the Featured Maps & Apps Group, Portal tries to open the view with an URL that looks like the following (you can hover on the op. view's thumbnail to see its actual URL):
https://<yourPortal.domain.com>/<webAdapter>/apps/dashboard-win/OperationsDashboard.application?open=<operationViewId> (if the version of your Portal is lower than 10.3, the URL will be slightly different)
At this point, Portal assumes that the Operations Dashboard desktop app has been deployed into the Portal machine's apps/dashboard-win folder (the physical location is typically c:\Program Files\ArcGIS\Portal\apps\dashboard-win if your Portal is 10.3 or above), and so Portal is trying to download the app from this deployed location into your machine so that you can install it and then use it to open the view. But from your description, it seems that Dashboard has not been deployed and so the URL results in a "not found" error.
Please also note that if you don't have Dashboard deployed and you try to open an operation view by clicking on the "Open in Operations Dashboard (Windows)" button on My Content page, Item Details page or the Sharing page, you will see the same "Page not found error" as they are all pointing to the same URL I mentioned above.
To deploy Dashboard into your Portal, you can follow this doc which has step-by-step instructions to show you how to do that.
If you have more questions, please post them back here or email the us at Dashboard4ArcGIS@esri.com.