CoreELEC is a linux operating system for ARM processors from the manufacturer AMLOGIC. You will never be able to run a CoreELEC virtual machine with an INTEL x86_64 processor. To do this you would need an ARM processor EMULATOR under INTEL x86_64. In the LibreELEC download section you have available a LibreELEC virtual machine that works with INTEL x86_64 processors with different operating systems (Windows, Ubuntu, …).