Downloading pre-made ISOs from torrent sites or file lockers is dangerous. They may contain malware, rootkits, or modified system files. Always create your own from an official Apple installer.
Then attach the ISO.
Since you cannot download a "ready-to-use" ISO from Apple, follow these steps to build your own safely: Mac Os Sierra Iso
If your goal is not a virtual machine but actually installing Sierra on a real Mac (e.g., a MacBook Pro 2011), you don’t need an ISO. Use the official installer to create a bootable USB: Downloading pre-made ISOs from torrent sites or file