Well I don't have a Steam deck but I know it uses an APU. Which is a GPU and CPU on the same die , what every laptop has. It might that the app doesn't recognise integrated GPUs to be powerful enough so it doesn't use them?

My understanding (potentially obsolete!) is that the integrated gpu of the Deck isn’t supported by, uh… whatever the AMD equivalent of Cuda was called, also there’s a chance you’d have to install it manually too.

You need amds proprietary ROCM driver which supports stuff like ai. However i already had issues setting up anything ai on my desktop fedora system, steam deck uses an immutable fs and afaik it gets reset after every update. If doing it on a desktop pc is hard then doing it on the deck will be like walking on lego. Good luck soldier.

So unfortunately I don't think ROCM supports the deck's APU, and it's too different for it to work by faking that it's a different GPU.

You can still do stable diffusion in CPU mode, but that will take 5-20 min per image (vs 7 seconds per image on my desktop's 6600 XT).

Overall it's not worth it unfortunately. As an alternative you might look into running stable diffusion through Google colab, which is a good way to get custom generation going for free.