Please post a backtrace, so we can see where the error is. Is it in az.plot_ppc or earlier?
Also, please reformat the code in your post using the preformatted layout operation – we can’t see what parts of the code are inside the with pm.Model() as model_particle: and which are outside, because the indentation has been lost.
I’m suspicious that this might be an old version of the library, because an up-to-date version would complain if particle_model wasn’t visible to az.from_pymc3.