This is an limitation of your DICOMs and not my software. The dataset is missing image instance 2, so the dataset has instances 1,3,4,...176. It is appropriate for dcm2niix to flag these partial datasets as they require human intervention. In this case, you can simply delete instance number 1, and then you will have virtually the full dataset (3..176) of contiguous slices. A simple way to sort DICOMs by instance number is to put the instance number into the filename. You can use the dcm2niix renaming feature ("r -y") with the instance number ("%r") in the filename to help. For example:

./dcm2niix -r y -f %t/%s_%p/%4r.dcm ~/Downloads/PPMI