You require a Windows 10 Pro or Windows 8.1 Pro License in order to be eligible for Downgrade Rights to Windows 7 Pro. See OEM Downgrade Rights for more details.
For a modern system with a UEFI BIOS you require updated Windows 7 Installation Media due to lack of native driver support. See either:
- Download a Windows 7 SP1 (Media Refresh 2011) Commercial OEM or Retail .iso and Add USB 3.0 Support
- Download a Dell Windows 7 SP2 (Media Refresh 2016) OEM Reinstallation .iso with Native USB 3.0 and NVMe Support
Finally you need to Optimise the UEFI BIOS Settings for Windows 7 64 Bit. See my guide Unified Extensive Firmware Interface (UEFI). Note the UEFI guide is optimised for Windows 10 64 Bit. You'll need to follow the exact same steps except that you will need to Disable Secure Boot and Enable Legacy ROMs.