Why do I get an error when changing or editing report permissions?
This is typically due to the owner of of the report being an inactive user.
If the report does not have views, please open it in Analytics and use Save As to overwrite it. This will change the ownership of the report to you, so you can then edit the permissions.
If the report has views, you'll need to save the report as another one, set the permissions, redo views and schedules, and remove the previous ones.