Hey Gabriele,
I am really sorry that delay of this reply,
By default behaviour, only Super admin and the discussion owner have the permission to accept as answer.
If you would like set your Moderatori user group to have this user permission, you have to modify on this file ->
JoomlaFolder\components\com_easydiscuss\themes\simplistic\post.qna.php
// LINE 20
<?php if( ( $reply->access->canMarkAnswered() && !$question->islock && $my->id == $questionOwner) || DiscussHelper::isSiteAdmin() ) { ?>
// Replace with
<?php if( ( $reply->access->canMarkAnswered() && !$question->islock && $my->id == $questionOwner) || $reply->access->canMarkAnswered() || DiscussHelper::isSiteAdmin() ) { ?>
Hope this help.