When you are modifying project items and their child elements, TestComplete can make a backup copy of project item files. For instance, when you modify script code and save the changes, TestComplete saves the older version of the script file to the file having the same name and the .bak extension. If TestComplete was unable to back up a file, it displays this error message to inform you about the error. Most possible causes for the error is that the .bak file already exists and it is locked by another application, or TestComplete does not have write permissions to save the file.