Apple just released an update patch for macOS Mojave 10.14.5 to fix issues related to Boot Camp. This problem mainly occurs on Mac computers equipped with Fusion Drive.
The macOS Mojave 10.14.5 Boot Camp upgrade patch is small, only 1.9 MB. This update addresses an issue which prevents the creation of a new Boot Camp partition on iMac and Mac mini with Fusion Drive.
Boot Camp Assistant is a multi-boot utility included with Apple Inc.’s Mac OS X that assists users in installing Microsoft Windows operating systems on Intel-based Macintosh computers. The utility guides users through non-destructive disk partitioning (including resizing of an existing HFS+ partition, if necessary) of their hard disk drive or solid state drive and installation of Windows device drivers for the Apple hardware. The utility also installs a Windows Control Panel applet for selecting the boot operating system.