Sounds like you are using Retroarch and the core selected is the 2024 core which is a game. You would need to change the core to the correct one for Atari 800 but it will take some extra steps to get working.
https://docs.libretro.com/library/atari800/
My advice would be to use the stand alone emulator Kat5200, it's a great emulator and is the easiest 5200/800 emulator to get setup: http://jillybunch.com/kat5200/