Windows 7 does not support Secure Boot and it must be OFF in order to Boot to Windows 7 installation media.
In the UEFI Boot Menu above some systems will allow you to select Change Boot Mode Settings to UEFI without Secure Boot or Legacy Directly: These system will use a UEFI Boot that always enables Legacy ROMs: Next look for the Secure Boot status (a handful of early UEFI systems don't have Secure Boot) such as the OptiPlex 790 boot menu below. If these terms are not present on the Boot Menu you have a Legacy only BIOS. Other OEMs may use different usually function key combinations and have slightly different BIOS General User Interfaces.
On a Dell system press when powering to enter the Boot Menu. The SLIC 2.1 can be passed through to a VMware Virtual Machine
I demonstrate creating the January 2020 Media Refresh Installation media with full driver support and automatic OEM System Locked Preinstallation for an OptiPlex 7040 (6th Generation Intel Skylake Hardware).
The Multi-Lingual Dell Windows 7 Pro (Media Refresh January 2016) or "Skylake" ISO may be downloaded using the Dell OS Recovery Tool and a valid Dell Service Tag. For a 2014-2016 computer you may get better performance installing a Linux Distribution for example Zorin OS 16. Windows 7 is not every day use, particularly internet usage as it is now insecure. Dell have not removed any downloads from their servers but Windows 7 is only recommended for essential legacy purposes.
Microsoft recommend installation of Windows 10 (if your computer is newer than 2014 featuring standard technologies like a UEFI BIOS and Secure Boot and has a Solid State Drive) or if your computer is older purchasing a new PC with Windows 11. Windows 7 reached end of life in January 2020 and Microsoft has consequently removed all ISO images and updates for Windows 7 from their servers.