In the documentation on Singletons in QML: https://doc.qt.io/qt-6/qml-singleton.html
in the section on defining singletons, it is stated that 'pragma singleton' should be added, but singleton word is written with a lower case letter and should be written with an upper case letter:

If you use a lowercase name, the qml file will not be built (error: Unknown pragma 'singleton')

It should be like this: