The problem is most emulators are default set to emulate the ZX Spectrum 128k. Jetpac doesn't work on the Spectrum 128k even on 48k mode that you can select on the 128k. This was due to a hardware bug with the 128k model that wasn't on the 48k model of the spectrum. The bug for some reason only affected about 5 ZX 48k games from booting on the 128k Model.
Pretty much every emu has an option to select which model of spectrum is emulated. Just select the 48k model and the game will boot.
I pretty much play Jet Pac on my 3ds every day using the ZXDS emu but sometimes get my ZX Spectrum 48k outta the cupboard to play it that way.
Model select screen in ZXDS and below pic is game running.
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