https://wccftech.com/microsoft-blocks-some-surface-book-2-upgrading-windows-10-1903/

Microsoft resolved quite a few issues and removed associated upgrade blocks over the weekend. However, a new Windows 10 1903 problem has been identified. The Windows maker has implemented another compatibility block that will affect some Surface Book 2 devices with discrete GPUs (dGPU).

The problem appears to be a compatibility issue between the new version and the NVIDIA GPUs used in some Surface Book 2 models. Microsoft explains: