You added another verb, "is" to the equation. And a "what." That confuses Duo.
despite that the resulting sentence would sound strange, is it grammatically possible that eso is the subject and usted is the object? "does that propose you?"
How about a sentence like "eso propone aquel"?
Is it possible for aquel to be the subect in that case? And is it then necessary to say "a eso propone aquel" to differentiate the subject?