Mover return para o final

Na função:

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let row = indexPath.row
        let meal = meals[row]
        let cell = UITableViewCell(style: UITableViewCell.CellStyle.default, reuseIdentifier: nil)
        cell.textLabel!.text = meal.name
        return cell
        let longPressRecognizer = UILongPressGestureRecognizer(target: self, action: #selector(showDetails))
        cell.addGestureRecognizer(longPressRecognizer)    
}

Quando o codigo passa pelo return ele sai da função, para resolver altere a função para:

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let row = indexPath.row
        let meal = meals[row]
        let cell = UITableViewCell(style: UITableViewCell.CellStyle.default, reuseIdentifier: nil)
        cell.textLabel!.text = meal.name
        let longPressRecognizer = UILongPressGestureRecognizer(target: self, action: #selector(showDetails))
        cell.addGestureRecognizer(longPressRecognizer)    
        return cell
}