您应该在类中的函数之外使用变量。

class ViewController: UIViewController {

   IBOutlet weak var minValueTxt: UITextField!
   IBOutlet weak var maxValueTxt: UITextField!
   //IBOutlet 3
   //IBOutlet 4

   var minNumber: Int = 0
   var maxNumber: Int = 0
   var appNumber: Int = 0

   @IBAction func generateNumber() {
      minNumber = Int(minValueTxt.text!)!
      maxNumber = Int(maxValueTxt.text!)!
      appNumber = Int.random(in:minNumber...maxNumber)
   }

//the rest of the code
}