Final answer:
The contract between Sidney and Bookie Bryan may not be enforceable due to illegality, as many jurisdictions have regulations or prohibitions against gambling.
Step-by-step explanation:
The missing element that might prevent Bookie Bryan from enforcing the gambling agreement with Sidney is legality. For a contract to be enforceable, it must be for a legal purpose and not be against public policy.
Since many jurisdictions have laws that either regulate or prohibit gambling, the contract might be deemed illegal if it was made in a jurisdiction where gambling is illegal or if it was not conducted in accordance with state regulations.
Without knowing the specific jurisdiction and legal context, it is difficult to give a definitive answer, but illegality is a common reason why gambling debts are often not enforceable through legal action.