Microsoft has been blocking Surface Book 2 users from updating to the May 2019 Update (1903), due to an issue that caused the dedicated GPU driver for the device to be dropped from Device Manager. The problem left SB2 users with having to use only the laptop's Intel integrated graphics.
Now however, after months and months of waiting, a fix is finally available, according to Neowin.net. Microsoft has released a set of firmware updates, that not only fix the display adapter issue, but improve battery "stability" and solve another issue involving unwanted CPU throttling.
Neowin reports that applying the firmware updates *should* allow users to then update to the May 2019 Update (or presumably even the recently gone final November 2019 Update), although it may take a bit to remove the block, as it's still listed as a known issue.
If you're a Surface Book 2 owner, you should check Windows Update for the new firmware releases, or you can manually download drivers and firmware packages for the breakthrough Surface device.