Microsoft says some Macs running Windows 10 may be blocked from updating to Windows 10 version 1903 because of a compatibility block triggered by older software or hardware. Microsoft also provides a workaround for users affected by this compatibility block, explaining that updating the Apple Boot Camp Windows Support Software drivers may allow them to install Windows 10 May 2019 Update. Microsoft says a resolution for this compatibility hold is currently being under development, with a possible solution for the issue being published in late July. There are still eight compatibility update blocks preventing Windows computers from receiving the Windows 10, version 1903 feature update.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-blocks-some-macs-from-updating-to-windows-10-version-1903/