Cannot find 'M' in scopeというエラーがでてしまう

import UIKit

class MainTableViewController: UITableViewController {
    
    private let cellId = "cellId"
    private let players = [
        ["山田","小林","田中","佐藤","卯類","竹富"],
        ["山田","小林","田中","佐藤","卯類","竹富"],
        ["山田","小林","田中","佐藤","卯類","竹富"]
    ]
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .white
        tableView.register(MainTableViewCell.self, forCellReuseIdentifier: cellId)
        setupNavigationBar()
        
        
    }
    
    private func setupNavigationBar() {
        
        navigationItem.title = "NBA Player"
        navigationItem.largeTitleDisplayMode = .always
        navigationController?.navigationBar.prefersLargeTitles = true
        
        //navigationの背景色を変更
        let appearance = UINavigationBarAppearance()
        appearance.configureWithDefaultBackground()
        appearance.backgroundColor = UIColor(white: 0.9, alpha: 1)
        navigationController?.navigationBar.scrollEdgeAppearance = appearance
        navigationController?.navigationBar.standardAppearance = appearance
        
        
    }
    
    }
    //MARK - tableViewの設定
extension MainTableViewController {
    
    override func numberOfSections(in tableView: UITableView) -> Int {
        return players.count
    }
    override func tableView(_ tableView: UITableView, heightForHeaderInSection section: Int) -> CGFloat {
        return 50
    }
    
    override func tableView(_ tableView: UITableView, viewForHeaderInSection section: Int) -> UIView? {
        let label = UILabel()
        label.text = "Aチーム"
        label.textAlignment = .center
        label.backgroundColor = .darkGray
        label.textColor = .white
        
        switch section {
        case 0:
            label.text = "Aチーム"
        case 1:
            label.text = "Bチーム"
        case 2:
            label.text = "Cチーム"
        default:
            break
        }
        
        return label
    }
    override func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
        return 60
    }
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return players[section].count
    }
    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: cellId, for: indexPath) as! MainTableViewCell
        
        //cell.textLabel?.text = players[indexPath.section][indexPath.row]
        return cell
    }
}