Bootable USB for Windows 7 isn’t showing up on MacBook Pro 12,2
The Boot Camp Assistant will not accept Windows 7 because your Mac is a 2015 model. Starting in 2015, Apple eliminated including a legacy BIOS with the Mac firmware. Windows 7 can only BIOS boot and therefore can not be installed on your Mac. It does not matter how you create the flash drive or what method you use in an attempt to install.