Are you using the "main" branch of this QR code sample? If you are, try switching to the "openxr" branch - microsoft/MixedReality-QRCode-Sample at OpenXR (github.com).

Note that the "main" branch of this sample is working with Unity's "Windows XR Plugin" which works with the WinRT APIs in in Unity 2019 or 2020 LTS versions.

After upgrading to Unity 2020 or Unity 2021, You can also use "OpenXR plugin" for HoloLens 2 developement. With OpenXR plugin, the app can use the built-in support for SpatialGraphNode, and the QR code tracking will work mostly the same way as above.

To view the OpenXR version of the QRCode tracking on HoloLens 2, please checkout the "openxr" branch of this sample repro, https://github.com/microsoft/MixedReality-QRCode-Sample/tree/OpenXR.