When you open a file, you get an error: "Could not complete your request because it is not a valid Photoshop document."
This can occur when you save a different file type, for example a JPEG, with a .psd extension in the file name (mydocument.psd).
Rename the file so that the file name extension matches the correct file type. (mydocument.jpg)
If you cannot remember the actual file type the file should be, see Determine the file type of a document.