I notice there are spaces in your file path. When this occurs, you will want to use quotes for terminal programs so that they are able to detect that you are not providing multiple arguments:

  C:\Users\rubin>dcm2niix "C:\Users\rubin\BIDS\sub-R0120062\func C:\Users\rubin\Documents 2021\Germany\fMRI data\Motor imagery dataset\DICOM\Files\R0120062\source data\func.PAR"

You may want to consider the terrific FSL training course, which has a lot of details for experts and beginners alike:
  http://fsl.fmrib.ox.ac.uk/fslcourse/
The preparatory material has a lot of details that can help you work with the command line
   http://fsl.fmrib.ox.ac.uk/fslcourse/#Preparatory
(You can run FSL on a Windows computer with the Windows Subsystem for Linux).

Andrew Jahn has some great videos as well:
  https://www.youtube.com/watch?v=9ionYVXU...