Startup disk doesn't show my windows partition
A previous answer, which will get Windows 7 installed on your Mac, can be found at Installing Windows 7 or 10 Pro 64 bit without DVD or Flash Drive.

If you have the Windows 7 product key, then you should make sure you have the latest Windows 7 iso download. You can get this from Download Windows 7 Disc Images (ISO Files).

I will point out that the Windows 7 product key can also be used to install Windows 10 for free. The latest Windows 10 can be found at Download Windows 10 Disc Image (ISO File). Instruction for installing Windows 10 can be found at How to install Windows 10 into a 2011 iMac without using the Boot Camp Assistant, an optical (DVD) drive or third party tools?.

Addendum

You could also probably install Windows 7 by first booting from a Windows 10 USB installer. This would then require using the CLI to install Windows 7 to an internal drive. The instructions would be similar to this answer. If interested, I could include updated instructions.