Hi Eleonora,
I didn't find any problem in running with the data you sent, but I note that:
- The mask that came is called mean_FA_skeleton_mask_dst.nii.gz, but the one in your command line (earlier email) is mean_FA_symmetrised_skeleton_mask_dst.nii.gz.
- The mask isn't really a mask, but a distance map. If the file that you actually used is similar, it's likely that there are no voxels left to run the test on.
Try to run again omitting the mask (-m); randomise will run the test only for the non-constant voxels, so the background will be ignored.