Hm, it seems impossible to respond to comments starting from sixth level...
When this translation came up, I immediately understood it in my mind as "You paid yesterday. I am crying today."
Which totally should be accepted! If Duo doesn't, everybody report, this sentence makes as much sense as any sentence can :D
The audio has плачу́ though.
Oh well, if you heard the audio then it breaks the mystery... But if you only see the sentence in a typing exercise, both should be accepted! Same as, let's say, "I like nuts" in English can be translated to "Я люблю горіхи" (the tiny hard fruits) and "Я люблю гайки" (the metal things), because "nut" means both and we have two words in Ukrainian for those things. Of course the first one makes more sense, but the second one is no less valid.
P.S. Didn't get what you meant with the "impossible to respond" part.
Yes, there are. When Duo gives you a Ukrainian sentence ("Я плачу сьогодні.") and you task is to type in the English translation, both "I cry today" and "I pay today" should be accepted.
P.S. Oh, I get it now, I just didn't get what "sixth level" was - the comment branch level :)