"She performed well."
How would you fit "well" into that phrase? "Έδωσε μια καλή παράσταση"; Everything switches from a verb-adverb clause to one with less emphasis on the verb, making it a much less than ideal translation, at least in Duolingo terms. In a real world situation, with fewer constrains, it is fine. perform
Δυστυχώς ναι, είναι λάθος. The focus is on "παράσταση", not the verb. I can think of examples , both in GR and EN, where it seems that when you choose to express a meaning by converting a perfectly working, single word verb to a verb clause that's phrasal, adding an adverb in the latter does not work. According to the Greek wiki verb page, when you go from verb to verbal phrase, the verb loses its lexical role and becomes simply an auxiliary, expressing only grammatical meaning. So, maybe that's why, while in general the adverb modifies the verb, the verbal phrase needs an adjective. The focus clearly shifts as the action is not described by the verb any more.
However, sometimes you could still move the focus to the verb with an adverb, as shown in the last two examples below. It still does not work with the exercise in question, but that is not a verb turned into a verbal phrase. (I think it's getting confusing...)
Κάνω (πολλή) δουλειά - δουλεύω (πολύ)
Δίνω (σωστή) απάντηση - απαντώ (σωστά)
Δίνω συμβουλή - συμβουλεύω
Βάζω (προσεκτικά) την υπογραφή μου - υπογράφω (προσεκτικά)
Κάνω (γρήγορα) την προσευχή μου - προσεύχομαι (γρήγορα)
You could translate them in English to see how many of them work either way and see if a rule emerges. :) Unfortunately I don't have much time to look into it in more detail to see if there is an explanation in English grammar. I hope this was useful in some way at least. (And I completely agree with the timer-no-pause-to-report issue.)