Windows 7 requires an optical drive to be connected during an install. If you don’t have one, here is a workaround.
Download Windows Image Tool
Download it here, pick the first one from the top. If the website is down, download it from my website below.
Modify the Image
Insert your flash drive with Windows iso already installed and launch the Windows Image Tool.
Pick your flash drive from the Destination Path (USB Drive) drop down menu.
Make sure this option is checked – Add USB drivers to an offline Windows 7 image. Now, hit the Start button.
It will take a while. After the program is done, Windows should install without an issue.