Ok, that’s good actually: this error means you’re sampling now 
I’m out of my league regarding the Windows and __main__ clause here though, so pinging @aseyboldt as he dug a lot about these issues 
(see Bruno’s penultimate message Adrian, and then the first error with a weird KeyboardInterrupt).
The ValueError: Mass matrix contains zeros on the diagonal is very probably because your prior is too flat for eta (so flat that there is no derivative). I think you’re taking an std of 20 and then exponentiating it, so this is really huge. Here are good recommendations for prior choices. hope this helps 