Boot Camp Assistant can't find installer disc.

I'm trying to partition my hard drive to put on windows. I'm using a 15 inch MacBook Pro from Mid 2009. I'm using version 10.11.6. This is the pop up I keep getting

Here are all the stats of my computer;

15-inch, 2.53GHz, Mid 2009

2.53 GHz Intel Core 2 Duo

4 GB 1067 MHz DDR3

Startup Disk is New Brain (a while ago I had the DVD reader replaced with a solid state drive)

NVIDIA GeForce 9400M 256 MB

I have tried all sorts of things to fix this, but nothing works. I made sure I downloaded the proper windows iso, I downloaded the most recent windows support, I made sure the external flash drive I'm using has enough storage and that it's formatted as a FAT32...nothing seems to work! Any advice would be greatly appreciated. Thanks! 🙂