Make sure you allow enough disk space inside the temp VM: apart from the requirements of the OS it is running, the 'Install OS X El Capitan' application is about 6.3 GB (plus the size of the disk image if you copy it into the VM). The reason this works is that the installer package on the disk image and the actual macOS installer bypass the model check if they detect they are running inside a VM. Once you have the Mojave VM, you can open the El Captain 'InstallMacOSX.dmg' file from inside that VM and run the installer, which creates the 'Install OS X El Capitan' application, copy that application out to the host then use it to create your El Capitan VM (and delete the temp Mojave VM if you don't need it for anything else). You can download the Mojave installer from Apple's support page (it comes as an application, not a disk image, so it can be used directly to create the Mojave VM). With Fusion Player and Fusion Pro, run nearly any OS as VMs on Mac for development, testing, gaming or even simulating production clouds on local desktops. It take just a few minutes for instant Launching. IT professionals, developers and businesses choose VMware Fusion desktop hypervisors for unmatched OS support, rock-solid stability and advanced features. If you don't have access to a suitable older Mac model, the workaround is to create a temporary VM for a later macOS which your Mac is able to run, e.g. Download the Mac OS X El Capitan 10.11 VMWare Image, you don’t have to spend even a day to get the El Capitan installed successfully on VMWare on Windows (Windows 10, 8,8.1,7,). If your iMac is a 2017 or later model, then it is too new to run El Capitan, so the installer package inside the disk image will refuse to proceed. The catch is that you can only do this on a Mac which is able to boot El Capitan. That application is the file you need to create an El Capitan VM.

dmg in macOS and run the installer package inside it, which will create an 'Install OS X El Capitan' application in your Applications folder. The key detail is that you can't use that InstallMacOSX.dmg file to create a VM.

This problem has come up before and I gave a more detailed answer at the time.

Mac os x vmware image 2017