Hi-
The error message is asking for the output file name to be provided differently---instead of an *absolute* path, like:
/A/B/C/file.nii.gz
... to use a relative path, so one that doesn't start with '/', like:
../D/B/C/file.nii.gz
So, perhaps try:
\mkdir tmp_ttest
3dttest++ -ClustSim -AminusB -prefix ./tmp_ttest/SN_ASDvsSSD.nii.gz \
-setA \
[setA list]
-setB \
[setB list]
--pt