The error message, minus the name, is 'cannot call member function ... without object'. That means you are attempting to call a non-static member function without an object. If it was a static member then it would not need an object.
The message isn't exactly clear but I think what you want there is :
QObject::connect(
ui->list
, &QListWidget::itemClicked
, ui->list_2
, &QListWidget::addItem
);
When passing a pointer to a function as an argument to a function no arguments can be used with it.