Can't use QtQuick.Controls

[w.tkm]

I can't use QtQuick.Controls.
I try to import this, and no error when you build it.
However, when I run this, I get the following error.

QQmlApplicationEngine failed to load component
qrc:/qml/main.qml:22:5: ?^ TestRect ?????p????????
qrc:/qml/TestRect.qml:2:1: "QtQuick.Controls" ???W???[?????C???X?g?[??????????????

MyCode:
main.qml

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
    id: mainWindow
    width: 640
    height: 480
    visible: true
    title: qsTr("Hellow World")

    TestRect{
        id: test
    }

}

TestRect.qml

import QtQuick 2.12
import QtQuick.Controls 2.12

Item {
    Rectangle{
        anchors.top: parent.top
        anchors.left: parent.left
        width: 200
        height: 200
        color: "red"
    }
}

What is the cause of this?
I use Qt5.15.2 and windows10

[J.Hilk]

@w-tkm have you considered upgrading to at least the latest 5.12 version?

[w.tkm]

@J-Hilk
No, I haven't considered it because it has been working fine until now.
Sorry, I made a mistake, it's "5.15.2" to be exact.
I created a new project with the exact same settings and the exact same code, and it worked.
Was there a problem with the settings that I missed somewhere?