$ swift --version 
Apple Swift version 5.2.4 (swiftlang-1103.0.32.9 clang-1103.0.32.53)
Target: x86_64-apple-darwin19.5.0

$ cmake --version
cmake version 3.17.3

$ python --version
Python 2.7.18

$ xcode-select --version
xcode-select version 2373.

let srcMat = Mat(uiImage: srcImage)

import UIKit
import opencv2

class ViewController: UIViewController {
    
    @IBOutlet weak var srcImageView: UIImageView!
    @IBOutlet weak var dstImageView: UIImageView!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        let image = UIImage(named: "サンプル画像")!
        self.srcImageView.image = image
        self.dstImageView.image = self.convertColor(source: image)
    }
    
    func convertColor(source srcImage: UIImage) -> UIImage {
        let srcMat = Mat(uiImage: srcImage) // ERROR!
        let dstMat = Mat()
        Imgproc.cvtColor(src: srcMat, dst: dstMat, code: .COLOR_RGB2GRAY)
        return dstMat.toUIImage()
    }
}

-[Mat initWithUIImage:]: unrecognized selector sent to instance 0x280554620
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[Mat initWithUIImage:]: unrecognized selector sent to instance 0x280554620'
*** First throw call stack:
(0x1b03e4300 // ... // 0x1b01d52dc)
libc++abi.dylib: terminating with uncaught exception of type NSException