QML - Cannot assign to non-existent property "style"

There are 2 types of Buttons in QML:

• Button Qt Quick Controls 2: https://doc.qt.io/qt-5.10/qml-qtquick-controls2-button.html
• Button Qt Quick Controls: http://doc.qt.io/qt-5/qml-qtquick-controls-button.html

In your case, you are importing the Qt QuickControls 2 Button: import QtQuick.Controls 2.3, and that Button does not have the style attribute.

If you need to use the style you must import:

import QtQuick.Controls 1.4

instead of:

import QtQuick.Controls 2.3

---

If you are using items from Qt Quick Controls and Qt Quick Controls 2 you could separate them using a namespace:

import QtQuick.Controls 2.3 as QQC2
import QtQuick.Controls 1.4 as QQC1

QQC1.Button {
    text: "A button"
    style: ButtonStyle {...}
}

QQC2.another_item_of_Qt_Quick_Controls2{
}

You can customize Qt Quick Controls 2 button by modifying its two visual items of background and content item:

https://doc.qt.io/qt-5/qtquickcontrols2-customize.html#customizing-button

import QtQuick 2.12
import QtQuick.Controls 2.12

Button {
    id: control
    text: qsTr("Button")

    contentItem: Text {
        text: control.text
        font: control.font
        opacity: enabled ? 1.0 : 0.3
        color: control.down ? "#17a81a" : "#21be2b"
        horizontalAlignment: Text.AlignHCenter
        verticalAlignment: Text.AlignVCenter
        elide: Text.ElideRight
    }

    background: Rectangle {
        implicitWidth: 100
        implicitHeight: 40
        opacity: enabled ? 1 : 0.3
        border.color: control.down ? "#17a81a" : "#21be2b"
        border.width: 1
        radius: 2
    }
}