This is a known problem. It results from two facts:
- The chipset on your main board has only USB 3 drivers
- The Windows 7 installer has not USB 3 drivers
You need to create a modified ISO that has USB 3 drivers included. If you bought your system new from an OEM with Win7 pre-installed, they should have (and likely did not) provided you with installation media adapted for this. Creating your own is a complicated process. Dell has published a web page that describes in detail on to do it. I did it, but I must tell you, it was challenging. I suspect most people will give up and just let Win10 wash over them.
The product of your efforts will be a USB device that has a revised Win7 ISO, with which you can install Win7 on your computer.
By chance, is this a Dell computer?