Qt6 is a big change so for compatibility reasons Qt does not provide this module, it will be provided in Qt 6.2 as this post: Add-on support in Qt 6.0 and beyond points out.
For Qt 6.2 we are planning to provide the following additional libraries:
- Qt Bluetooth
- Qt Data Visualization
- Qt Lottie Animation
- Qt Multimedia
- Qt NFC
- Qt Positioning
- Qt Quick Dialogs: Folder, Message Box
- Qt Remote Objects
- Qt Sensors
- Qt SerialBus
- Qt SerialPort
- Qt WebChannel
- Qt WebEngine
- Qt WebSockets
- Qt WebView
(emphasis mine)
So you will have to wait for that release that according to what was announced will be available for September 2021