In the support documentation, Microsoft pointed out that due to compatibility issues,WindowsThe 10 May 2019 update may not be installed on older Macs.
Microsoft said that affected users may encounter compatibility tips, "Mac HAL Driver - machaldriver.sys: Your PC has a driver or service that is not ready for this version of Windows 10."
According to this document, Mac devices prior to 2012 or newer Mac devices running older versions of Boot Camp or Windows supported software drivers cannot download or install Windows 10 1903.
"Specifically, MacHALDriver.sys will be affected by the driver device on September 24, 2011 at 01:57:09 or earlier," Microsoft pointed out. "Updating the Boot Camp Windows support software driver may allow you to update to Windows 10 version 1903. You can check for updated Windows support software.
Microsoft said it is working hard to resolve this issue and is likely to lift the upgrade limit in the near future.