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)
You may have saved a file with the wrong extension, for example a JPEG file with a .psd extension. You cannot open the file in Photoshop due to an error
If you cannot remember the actual file type the file should be you can use the following method to determine the true file type of a document
Selecting File > Get Info... from the macOS Finder will not accurately tell you the true file type of a document as it only looks a the file name to make the determination. The method outlined below using the Terminal reads into the file header to get the true file type.
In Terminal type "file " (the word 'file' with a space after it)
Drag the file you want to determine the file type of onto the Terminal window
The path to the file and its name will appear in the Terminal window
Hit the Return key to see the file type