You use wrongly the command… still not clear why you write snap as a first parameter… the command should be launch as follows:
mt_prep_gamma 20171212 /media/datadrive/SAR/PSI_SBAS/SANTIAGO/INSAR_master_date/ 0.4
note that there is no snap written on this command. Still not clear why you included it
and please modify the command mt_prep_gamma as mentioned on the thread :Snap2stamps package: a free tool to automate the SNAP-StaMPS Workflow - #7 by mdelgado
It should work.
Please let me know.