Reproduce procedure:

1. Create a new Qt for MCU project from QDS
2. Enable Kit: Desktop Qt 6.3.1
3. Add a Quick Controls item, e.g. a Button, in Screen01.ui.qml
   1. import QtQuick.Controls 2.15
   2. Button { id: button}
4. Run

There is an error message:

QQmlApplicationEngine failed to load component
file:///C:/_dev/Brose_test_3/Brose_test_3.qml:8:5: Type Screen01 unavailable
file:///C:/_dev/Brose_test_3/Screen01.ui.qml: module "MCUDefaultStyle" is not installed
qml: Did not load any objects, exiting.

Workaround1:

Change the Kit to Desktop Qt 5.15.5

Workaround2:

In the auto generated file qtquickcontrols2.conf, comment out the line:

[Controls]
#Style=MCUDefaultStyle