I checked the sentence and εφτά is accepted. Please check again and see if there's a different problem with the sentence.
Because either you are being asked to translate from Greek into English - in which case 'seven' would be the correct translation - or you are being asked to write what you hear - in which case 'επτά' would be the correct answer (since the voice does not say 'εφτά').
Εφτά is the medieval writing/pronunciation and επτά the ancient one. Same for οχτώ and οκτώ or εννιά and εννέα.