That's not how the imperative is used. :) The imperative is used to give a command while the indicative is used for statements.
Έχετε is indeed second person plural. The imperative mood requires that no personal pronoun is used before the verb and in Duolingo is marked with the exclamation point at the end of the sentence to distinguish it from a simple statement that might not start with the personal pronoun, as you can in fact drop pronouns in Greek even in the indicative. :)