Bootable USB device
You’ve placed the Win7 ISO onto a USB stick, but you might need to follow additional steps to boot from it successfully.
Ensure the drive is set up with MSDOS MBR formatting
If you're unsure, try WinUSB maker. It's free and I've used it regularly without any problems.
You should open the tool, choose the ISO, and connect it via USB using the app. That’s it.