afni.nimh.nih.gov

The error states that you need to have a short integer
dataset in order to run 3dIntracranial. So what type of
dataset do you have? The output of 3dinfo might be
helpful.

I'll guess that you have a float-type dataset, and you
just need to convert it to short to use 3dIntracranial.
If so, try something like:

3dcalc -a input_dset+orig -expr a -datum short -fscale -prefix output_dset

- rick