As the message says, this is reporting the log probability (logp) of the initial value for beta under your specified prior
My guess is that np.log(traindf.days_since_last_order+0.01)) is allowed to be negative (indeed you add 0.01, so i guess there are zeros and log(0.01) < 0. So mu is negative, and the Poisson is undefined.
You should use pm.math.exp(mu) instead of just mu in the Possion. Reading a bit more carefully, I see you exponentiation inside the outer product, but it’s probably safer to work entirely in the latent space until the last moment