On its support page for the issue, Microsoft reported:
Mac devices prior to 2012 or newer Mac devices with the older Apple Boot Camp or Windows Support Software drivers have a compatibilty hold in place and are unable to update to Windows 10, version 1903. Specifically, devices with MacHALDriver.sys dated September 24, 2011 01:57:09 or older in Windows\system32\drivers are affected.Microsoft went on to state that updating your Boot Camp Windows Support Software drivers may allow you to update to version 1903 of Windows 10. The company is working on an official solution that it hopes will be out by the end of July.
Boot Camp is a utility on macOS that allows users to install and use Microsoft Windows operating systems on Mac devices.
You can read the full support doc by heading to Microsoft's website.