Windows 10 via Bootcamp MBR Partition Error
If you changed the file within Boot Camp that allowed you to install from a USB on an older version of the Mac don't bother, it won't work.

If this problem persists just burn the ISO to a blank disc. I used a DVD-R. Then install it from the disc. I recommend restoring the partition to a single partition and uninstall windows with the BCA and try again this time with a disc that has enough space for the ISO.

The gdisk solution in some cases will help for other individuals but for your case I believe you have an old Mac.