Boot Camp Error - "Error Loading Operating System"

Hi Everyone:

After scrolling dozens of support threads (am I going to get a response form Loner T?) and youtube videos, I can't seem to figure this out. I have a late 2012 iMac (running HS 10.13.4) and just recently installed a new SSD in place of the SATA. I am trying to get Windows 7 on via Boot Camp (I had it previously), but after the drive is partitioned and the boot camp assistant forces the restart (for installation of W7), I get the "Error loading operating system." error message on an otherwise entirely black/blank screen.

I tried the process a bunch of times - redoing the partition, reinstalling the iso to a flash drive, etc. but I can't seem to get this to start the windows installation for some reason. It seems as if it is having trouble locating the iso installation file as I cannot even get to some sort of installation screen. This happens when I force restart and boot into the usb file with windows.

Any help would be much appreciated! Thank you.

-Scott