Welcome!
A couple of things. First, what version of PyMC are you using? It looks to be an older version based on some of the argument names you are using. Second, you seem to have a variety of typos and other odd mismatches in your code. For example, you pass degrees as the nu argument, which I assume is supposed to be degress_freedom. And higher_boundhigher_bound isn’t syntactically valid. Finally, if you find yourself needing 20,000 samples and/or 35,000 tuning samples, something is almost certainly wrong with your model.
To your specific question, can you provide the full traceback? I ran a corrected version of your model on some toy data and everything samples just fine.