'Mustn't' in the Bible

But the Philistine leaders were angry with him, so they pleaded with him, "Send the man back! Let him return to the place you assigned him. He mustn't go into battle with us. Otherwise, he may become our adversary in the battle! How could there be a better way for this fellow to reconcile himself with his lord? Wouldn't it be with the heads of these men?

Achish answered David, "I know that I'm pleased with you. You're like an angel of God. But the Philistine leaders have said, "He mustn't go into battle with us.'