Typically you get this error when you attempt to convert a localizer (scout) scan, where a single DICOM series includes different slice orientations. The NIfTI standard requires that all slices in a volume share the same angulation. For most neuroimaging tasks we discard the localizer scans, which are only used to plan subsequent images. If this is your intention, you can ignore this error, or you could use the `-i y` option to ignore localizer scans. If you do need the information from the localizer scans, you will want to retain these images. 

For more details, see issue 894

https://github.com/rordenlab/dcm2niix/issues/894