Mac Boot Picker won't read my boot camp USB.

I recently bought a Late 2011 Macbook Pro 13"(Macbook Pro8,1), and I've had MacOS Lion installed on it. I have a second SSD inside my macbook, and I've been trying to install Windows 7 through it. But, After I make my bootable USB via Bootcamp on Lion, the macOS boot picker refuses to read my USB. What can I do? [edit] For some reason my macbook reads Windows 10 USB files from the official windows installer, but nothing else. Rufus, UNetbootin, and BalenaEtcher burned USBs with windows on them won't work. Linux ISOs work, though.