Does a divorce occur if a husband tells his wife, "If you enter the guest room, you are divorced," if his intention was to threaten and prevent her from entering, and she did not enter it? And is an expiation for breaking an oath sufficient in this case?

What your husband uttered is a conditional divorce based on your entering the guest room. The majority of scholars hold that a conditional divorce cannot be revoked and takes effect if the condition is met, regardless of whether the husband intended to enact the divorce or merely to prevent the action. This is the established ruling. If your husband sought a religious opinion from someone who advised him that expiation is necessary, or if he acted upon this by following a specific school of thought, then there is no blame upon him, and there is no harm then in your entering the room. It is important for spouses to communicate and to be cautious about being lenient with divorce pronouncements.