I have installed the only existing Atari ST core 'Hatari' and the TOS Image 1.02 US (United States region) which seems to be the only TOS image supported/needed by the Atari ST core. I named it 'tos.img' and put it in the Retroarch/system folder and the core recognizes it successfully.
But when I load 'Mission Elevator.st' game file for example Retroarch just terminates.