If your existing Windows 10 VM is set up for BIOS firmware and a MBR formatted disk, it’s understandable that your VM may not boot. There are things you have to do to to your VMs Windows installation (not Fusion related)  to change it over to UEFI and GPT disks in preparation for a Windows 11 upgrade. Web search for instructions on converting a Windows 10 system to UEFI firmware and GPT formatting.

If you’re not up to that, clean install a new Windows 11 VM and migrate your data. .