I can not provide much more advice without seeing the image, but this error usually suggests a file permission error. I would suggest you run 'ls -l "filename.nii"' from the command line. You will want to make sure that you have read access permission for the file. Often you can fix this with a "chown" or "chmod" command.

Again, running 'fslhd "filename.nii"' from the command line is likely to provide more feedback (assuming you have installed fsl).