Hi,
You install ESXi on the bare metal, so you first have to WIPE your macOS so it isn't what you think it is.
You can install ESXi within Fusion which means you can run a virtual machine in your virtual machine, but I doubt that is what you are after.
ESXi is also called "vSphere" just download that, but beware that if you install it on your mac that it will wipe out OSX.
VMware Fusion is what you want.
--
Wil