In order to take a screenshot of a screen and save the captured screenshot to library, use the below steps:

1.  Add the "Privacy - Photo Library Usage Description" key to info.plist

  <key>Privacy - Photo Library Usage Description</key>
  <string>YOUR CUSTOM USAGE DESCRIPTION GOES HERE.</string>

2. In your controller add the below function to take screenshot:

   func captureScreenshot(){
            let layer = UIApplication.shared.keyWindow!.layer
            let scale = UIScreen.main.scale
            // Creates UIImage of same size as view
            UIGraphicsBeginImageContextWithOptions(layer.frame.size, false, scale);
            layer.render(in: UIGraphicsGetCurrentContext()!)
            let screenshot = UIGraphicsGetImageFromCurrentImageContext()
            UIGraphicsEndImageContext()
            // THIS IS TO SAVE SCREENSHOT TO PHOTOS
            UIImageWriteToSavedPhotosAlbum(screenshot!, nil, nil, nil)
  }

     The first time you launch the app and call this method, the iOS device will ask you the permission to access the photos.