What I am finding is that - as long as your noun/pronoun are using the correct verb form, DuoLingo will accept it. It's when you are using the wrong verb form for the noun you choose that you get it wrong. So both "tu entri" and "voi entrate" are correct. However, if you had used "tu entrate" or "voi entri" then it would have been wrong. Does that help?
It's not a guess. Leanne's comment is correct. You have to make a decision whether to use Tu or Voi, that's true, but as long as your verb form agrees with your noun it will be accepted. Either tu entrate or voi entri will be accepted as correct, but it's up to you to decide which you want to write.
I'm not sure about you but for the translation of "You enter.", I have these words to choose from.
• uso • Voi • entrate
• passi • chiama • lavorano
Thus, it's easy to choose "Voi entrate." for the translation of "You enter." as "entrate" is the only one that is 2nd person plural conjugation of a verb.
It is a good habit to refer to the conjugations as first, second or third person, and singular or plural; it will get pretty 'exciting' when you get past the indicative present conjugation s. :)
Anyway, I suppose if I were to type out the translation, I might just type "Entri" or "Entrante" (upper case first letter).
Well, I might also type any of the following for the translation of "You enter."
• Tu entri.
• Lei entra. ( formal )
• Voi entrate. [ You (all) enter. ]
Hope that helps.
Happy e-learning on Duolingo!