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Unable to install Startup Disk Creator as download failed: E: http://cn.archive.ubuntu.com/ubuntu bionic/main amd64 usb-creator-gtk amd64 0.3.5 is not (yet) available (Unsupported proxy configured: 127.0.0.1://37809)
I faced the same problem.
Try to manually install it on command line sudo apt-get install usb-creator-gtk.
The application is installed and you can find it on "Show Applications", or manually execute it via usb-creator-gtk.
sudo apt-get install usb-creator-gtk
Required, but never shown