I am sorry to hear about the issues you are experiencing.
Would it be possible for you to provide some more details when the issue started appearing? According to the error message it seems that you are trying to delete an item which has already been deleted or could not be found. Could you please clear the browser's cache in order to make sure that the issue is not related to caching? You can also try to restart the application and then test the behavior.
If the issue still persists, can you please delete the error log files under ~/App_Data/Sitefinity/Configuration/Logs
folder and then try to reproduce the issue again and send the files generated under the above folder? This may give us additional information on what might be causing the issue.
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Sitefinity CMS Ideas&Feedback Portal
and vote to affect the priority of the items