"Why could Macbeth not say amen when he heard a voice say "God bless us"?
In Act 2, sc. 2, Macbeth asks his wife
But wherefore could not I pronoune "Amen"?
I had most need of blessing, and "Amen"
Stuck in my throat.
The word "amen" comes from the Hebrew and expresses warm approval of something that has been said just before. Macbeth would like God to bless him too, but cannot bring out the simple word "Amen." It stuck in his throat at the time, and all the words "'Amen' stuck in my throat" should be uttered by the actor in a hoarse rasping voice.
Macbeth does not understand why he could not pronounce that word, and his wife really doesn't care because she has more practical matters to think about; but the audience understands that God will not allow him to pronounce it because he has committed an unpardonable sin. Later, in Act 3, sc. 1, when he is planning to have Banquo murdered, he confirms that he has sold his soul to the devil for nothing:
For them the gracious Duncan have I murdered,
Put rancors in the vessel of my peace
Only for them, and mine eternal jewel
Given to the common enemy of man
To make them kings, the seeds of Banquo kings.
It was really very inappropriate and presumptuous for Macbeth to be saying "Amen" to one of the grooms' "God bless us" while he is standing there with his "hangman's hands" covered with Duncan's blood. It is like asking for forgiveness practically at the same moment he is committing the crime and when he should know he has committed an unforgivable sin. He is foolishly and vainly hoping that somehow everything will come out all right.