Now click on the Motherboard tab and unselect Floppy and make sure that IHC9 is selected in dropdown.Here you will find three tabs click on the Drag slider in front of processor to 2 and make sure that Enable PAE/NX is enabled.
Choose “Use an existing virtual hard disk file” and click on the folder icon to choose the file on desktop from downloaded OS X Sierra to install Mac OS on VirtualBox.