Try adding noise to your generated data. Deterministic processes are degenerate: you’re asking the model to sample from a distribution with sigma=0, which is not a distribution. For model testing, I recommend you generate data from your model itself (using pm.sample_prior_predictive), then use pm.observe to see if you can recover the parameters in a given prior draw. See here for an example. This procedure will help you avoid problems like these.
Also setting cores=40 does nothing, the number of cores is used only up to the number of chains.