afni.nimh.nih.gov

You need to resample the mask so that it is on the gridset as the functional data.

3dresample is one program that can do this for you.

3dresample -master func_task_pp_ICAaroma_bptf.nii.gz -prefix SN_resam.nii -input SN.nii

here the -master dataset is the one you want to match.

3dfractionze can also do this and will give you much more control over process.