Cannot find type "..." in scope

When I select line 8, I get

QRSkannerController

instead of

QRScannerController

Why a : after class ?

class:QRSkannerController:UIViewControlle   @Cannotfindtype'QRScannerController'inscope

It is probably not the exact code. You cannot define a class in its extension…

So please post real code and we'll see what happens.

Declare your QRScannerController class first...
...then add the extension code, like this:

class QRScannerController: UIViewController {
    var video = AVCaptureVideoPreviewLayer()
    var qrcodeFrameView = UIView() // this needs to be initialized to something
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // creating session
        let session = AVCaptureSession()
    }
}

extension QRScannerController: AVCaptureMetadataOutputObjectsDelegate {
    
    // add delegate methods here...
}

You have to define the QRScannerController. Since you added your class inside an extension, it the compiler won't be able to find what QRScannerController is.