This is not a problem, the .DS_Store is a hidden file on MacOS that stores information about the folder. It is not a DICOM image. Therefore, dcm2nii is correct in reporting that it will not convert that file. In theory, one should be able to detect DICOM files by looking for the signature 'DICM' 128 bytes into the file, but this would fail to detect some Siemens DICOM meta data, so my software attempts to read a file values before giving up.

However, I suggest you heed the warning displayed when you run dcm2nii suggesting you should upgrade to dcm2niix. dcm2nii is now very old, and may not deal with recent vendor images. You can get dcm2niix from Github (https://github.com/rordenlab/dcm2niix/re...) or included when you download with MRIcroGL. MRIcroGL has a user interface for converting images (Import/ConvertDicomToNifti menu item) that will display the command line arguments for calling dcm2niix. Alternatively, since you use Matlab you may want to try out Xiangrui Li's excellent dicm2nii converter. One handy feature of dcm2niix is that it skips hidden files such as .DS_Store.