For whatever reason it is expecting a file or directory named "Files" inside your current directory, but is not finding it. We cannot tell from the screenshot whether you have such a file or directory name.
I suspect that you have a directory that starts with 'Files' and then has a space in it, and that the command that was given to mex failed to put quote marks around the directory name. mex normally considers spaces to be the end of the option or file name.
