I read your command, I think you are specifying 2 files as inputs.
There should be 3 files. If you are smoothing cortical thickness, then you should specify the input file twice and then the output file.
first file contains geometry
second file contains the function that needs to be smoothed
third file if the output file
fourth argument is the amount of smoothing.
If that doesn’t work, can you share one file with me using dropbox and share the exact command that you are trying so that I can try to reproduce the error?