"You can't find your dictionary?"
Translation:¿No encuentras tu diccionario?
The reflexive form encontrarse does not mean to find but to be situated.
Treat regular and reflexive verbs as though they are different words with different meanings.
Besides that, you don't need puedes in a sentence like this. That might be a surprise, but that is what Duo is trying to teach in this lesson.
There are two solutions in the database with ustedes. They both start with the pronoun.
¿Ustedes no [logran / pueden] encontrar su diccionario?
¿Ustedes no encuentran su diccionario?
I agree with Shoshi that it is best to leave it out. Most Spanish-speakers omit subject pronouns most of the time.