The Legacy option si gone forever, due to incompatibility with new processors.
If you wanted to boot with legacy to use an old software, the best thing to do is :
- install windows 10 UEFI, forget Legacy
- install VMware player or VMware Workstation on your Windows 10 environment
- install Windows 7 on a new VMware virtual machine
- install your old software on this Windows 7 virtual machine
- enjoy your old software