I am unable to download Windows on my Mac using Bootable USB

I'm currently trying to download Windows onto my Mac, but Boot Camp won't allow me to continue with a USB. When I have a USB plugged in, after I click continue past the first page on Boot Camp I get a message that reads: "External storage device attached. Please remove any external storage device attached to this system before continuing." I've tried to download Windows without the USB as well by just choosing my partition size and ISO on the next page, but when I try to install it, I get the message: "Can’t install the software because it is not currently available from the Software Update server." I've tried troubleshooting these issues every way I possibly can, but I haven't seen anyone else have the issue of Boot Camp not even letting them download Windows onto a USB. Does anyone have a fix for this? My USB is also connected by an adapter - is it possible that this is the issue?

[Re-Titled by Moderator]