首先，private修饰符是您的问题。
变更

private set(value){field = value}

至

set(value){field = value}
//public by default

否则你不能在课堂之外使用setter。阅读这里。
对于在类中声明的成员：private表示仅在该类内部可见（包括其所有成员）;
其次，你误解了一些东西：

var kon = person.someProperty
 kon = "manulilated"

在这几行中，你并没有改变对象的属性，在创建变量kon之后，作为一个指向someProperty的String，你将这个局部变量重新赋值给其他变量，这个赋值 * 不等于改变 * person.someProperty的值！它对对象完全没有影响。

someProperty具有私有setter。当setter为私有时，不能在类外对其进行设置

这是一个尝试用Java风格编写Kotlin的好例子。
这是如何在Kotlin促进语言能力。人们可以看到有多少更短和更清楚的代码。个人类只有3行。
使用数据类使我们不必定义hash、equals和toString。

fun test() {
    val person = Person("john", 24)
    var kon = person.someProperty
    person.someProperty = "crap" //this doesn't allow me to set value
    kon = "manulilated"  //this allows me to set the value (yeah, but not in the class, this is a local string)
    val sample_text = SampleText("${person.name}${person.age}")
    sample_text.text = kon

    println("Person = $person")
    println("Kon = $kon")
    println("Sample Text = $sample_text")
}

/** class for person with immutable name and age
 * The original code only has a getter which allows to use 'val'.
 * This can be set in the constructor, so no need for init code. */
data class Person(val name: String, val age: Int) {
    /** The field value is converted to uppercase() (capitalize is deprecated)
    * when the value comes in, so the get operation is much faster which is
    * assumed to happen more often. */
    var someProperty: String = "defaultValue"
        // setter not private as called from outside class
        set(value) { field = value.uppercase() }
}

/** simple class storing a mutable text for whatever reason
 * @param text mutable text
 */
data class SampleText(var text: String) {
    override fun toString(): String {
        return text
    }
}

这就是结果：

Person = Person(name=john, age=24)
Kon = manulilated
Sample Text = manulilated