Why can't we assume the Twelve knew about Jesus' death and resurrection in Acts 3?
They could not be expected to know it because faith comes by hearing the revealed Word of God, not by assuming what God has not yet spoken. In Acts 3, the Lord had not yet revealed the change from law to grace, so Peter and the Eleven were not accountable to believe what had not been made clear.
Les explains that people often ask whether the Twelve knew Jesus was going to die and be raised from the dead and could make it a gospel. The answer is no, you cannot assume what God is going to say; you must wait until He has spoken it and then believe it. Deuteronomy 29:29 says the secret things belong to the Lord, but the revealed things are for us. Romans 10:17 says faith comes by hearing, and hearing by the Word of God. God's Word is not a vague mishmash floating in the ethereal; it is what God has actually spoken. In Acts 3, the Lord had not yet said anything about shutting down the temple, no longer being under law, or being under a new manifestation of God's grace. So He did not expect Peter and the Eleven to believe what He had not said, and they were not negligent in their duty.