This error has the following cause and solution:
You added a file from a folder other than your project folder (or one of its subfolders), and Visual SourceSafe cannot determine which project corresponds to your new file's home folder.
Visual SourceSafe must map the local copy into a Visual SourceSafe project path for this operation to work. Copy the file to a subfolder at or below your IDE project. If the file is already in Visual SourceSafe in a different project, share the file to your current project, and try the operation again.
You will continue to get this message for the file as long as the name cannot be mapped. If this happens, you should remove the file from your project file, copy or share it, and then re-add the file.