From another comment of mine on the subject:
First test out your USB drive to ensure its functioning correctly.
Download this tool from mobatek. It's free.
I recently had the same issue when "downgrading" a shitty windows 10 laptop to windows 7 for my gf. I was doing everything correctly, the USB was just configured incorrectly.
Edit: here's a guide using it, even though I think it's straight forward.
First test out your USB drive to ensure its functioning correctly.
Download this tool from mobatek. It's free.
I recently had the same issue when "downgrading" a shitty windows 10 laptop to windows 7 for my gf. I was doing everything correctly, the USB was just configured incorrectly.
Edit: here's a guide using it, even though I think it's straight forward.