If you change to 48k machine type in the RGUI, then Jetpac will run. By default, lr-fuse, which I assume is you are using, runs the the 128k machine - you will see the line fuse_machine = 'Spectrum 128K' in the file /opt/retropie/configs/all/retroarch-core-options.cfg. According to World of Spectrum, Jetpac runs on a 16k machine.
