Almost, but not quite. You agree upon something - agree upon <noun> We agree upon a solution "agree" can take number of different endings, or it can be expressed with no ending, when what the agreement is about is understood "We agree that the mistreatment of animals should be stopped." or "I think that the mistreatment of animals should be stopped." "Then we agree." [that the mistreatment of animals should be stopped]

Yes, you have to specify what you are agreeing upon, when using 'upon'. If you simply say, "yes, we agree upon", my immediate question is "agree upon... what?"

Upon is a preposition, meaning it needs an object. All prepositions, including upon, up on, around of, inside of, etc., need a target.