"Did you guys understand this movie?"
Translation:¿Ustedes comprendieron esta película?
In the generic Latin American Spanish taught by Duolingo, there is only one level of formality for the plural 2nd person.
In Spain, you could use "vosotros" in place of "ustedes" for informal situations.
I'm old and British and would never use "you guys or you all". You seems fine.
Entendisteis es sinonimo a comprendieron... Y entendisteis no me lo acepta
Looks like you may have a typo -"vosotros"- but the suggestion is valid. "You guys" is pretty informal. While in Latin America "vosotros" is not used, in Spain, it would seem more appropriate.
Is "ustedes chicos comprendieron esta pelicula?" correct? It's not accepted.
Is there an alternative to Duolingo based on Castilian Spanish and not generic Latin American Spanish, which in any case is a Duolingo invention? Alternatively, why can't Duolingo adapt to the extent of accepting Castilian answers, as it accepts both US and UK English words, eg movie = film.