I’ve downloaded it, and I’ve got all the bios files in the retorarch/system directory.
But all it does is say that it needs a real bios.
Core info says the 5200.rom is present and correct as well, (I’ve also got all the atari os for 800 in there as well for good measure as I play this too)
I have 5200.rom, ATARIBAS.rom, Atariosa.rom, ATARIOSB.rom & ATARIXL.rom