Final answer:
Elizabeth wants her husband to testify in court that she always tells the truth. Proctor can't prove his allegation because it's a personal matter without evidence or witnesses.
Step-by-step explanation:
Elizabeth wants her husband to say that she never lies and is always truthful in court. She expressed this desire in Act III of Arthur Miller's play The Crucible. The reason Proctor can't prove his allegation is because it's a personal matter and there is no concrete evidence or witnesses to support his claim.