I don't know what this means: "This has not worked between us." I tried "This has not worked for us," but that was marked incorrect. Could someone please help me understand this?
He could be referring to a failed relationship, partnership, mutual endeavor, or the like. These practice sentences tend to lack much specific context that would otherwise help constrain a more specific, overall interpretation/meaning/conception. So this sentence could be used in any number of communication situations.
"This has not worked for us" is incorrect because בינינו means "between us", not "for us". I agree that this sentence needs context duolingo doesn't provide in order to make sense.