Why can't I access my ListView in the MainActivity file?

I created a ListView in my activity_main.xml file like this:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <ListView
        android:id="@+id/listview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:layout_editor_absoluteX="16dp"
        tools:layout_editor_absoluteY="15dp" />
</androidx.constraintlayout.widget.ConstraintLayout>

I'm trying to populate it like so in my MainActivity.kt file:

var adapter = ArrayAdapter<String>(applicationContext,android.R.layout.simple_dropdown_item_1line,post)
                listview.adapter = adapter

However, I get an error on listview and it says that it has an Unresolved reference. I am following a tutorial and they didn't have this issue. It might just be that the tutorial is a bit date (it was made in April 2020) and accessing layout variables has changed.

Any suggestions?