Module VirtualKeyboard not available in Qt Quick UI-Prototype? Err-Msg: not installed

Question

I want to create a Qt Quick UI. So I started a new Qt Quick UI-Prototyp-Project. I got standard template but compilatoin already fails with Err-Message.

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.VirtualKeyboard 2.15

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

    InputPanel {
        id: inputPanel
        z: 99
        x: 0
        y: window.height
        width: window.width

        states: State {
            name: "visible"
            when: inputPanel.active
            PropertyChanges {
                target: inputPanel
                y: window.height - inputPanel.height
            }
        }
        transitions: Transition {
            from: ""
            to: "visible"
            reversible: true
            ParallelAnimation {
                NumberAnimation {
                    properties: "y"
                    duration: 250
                    easing.type: Easing.InOutQuad
                }
            }
        }
    }
}

 file:///.../VKTest/VKTest.qml:12 Type InputPanel unavailable qrc:/QtQuick/VirtualKeyboard/content/InputPanel.qml:127 Type Keyboard unavailable qrc:/QtQuick/VirtualKeyboard/content/components/Keyboard.qml:38 module "QtQuick.VirtualKeyboard.Plugins" is not installed

But creating new Qt Quick Application with standard template - equal to UI-Prototype template - compiles fine.

import QtQuick 2.15
import QtQuick.Window 2.15
import QtQuick.VirtualKeyboard 2.15

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

    InputPanel {
        id: inputPanel
        z: 99
        x: 0
        y: window.height
        width: window.width

        states: State {
            name: "visible"
            when: inputPanel.active
            PropertyChanges {
                target: inputPanel
                y: window.height - inputPanel.height
            }
        }
        transitions: Transition {
            from: ""
            to: "visible"
            reversible: true
            ParallelAnimation {
                NumberAnimation {
                    properties: "y"
                    duration: 250
                    easing.type: Easing.InOutQuad
                }
            }
        }
    }
}

but with main application:

#include <QGuiApplication>
#include <QQmlApplicationEngine>


int main(int argc, char *argv[])
{
    qputenv("QT_IM_MODULE", QByteArray("qtvirtualkeyboard"));

#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
#endif

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);
    engine.load(url);

    return app.exec();
}

Does anybody also had this problem or know a reason? I tried it witha clean Ubuntu+QT5.15 (QtVirtaulKeayboard checked) and Windows. Maybe Virtual Keyboard doesn't work with a UI-Prototype?