What does Completed But Error or Completed, But Error mean?

It means the request worked (200 OK response) and didn't encounter any error like 302, 301, 501, 404 however results could not be found.  Generally this happens due to a search engine changing their format and a marker can not be found.

Check to make sure you are using the latest version of Scrapebox and/or the addon/plugin that you are getting the error in, as often times it is already fixed in a new version.