https://docs.fedoraproject.org/en-US/Fedora/24/html/Installation_Guide/sect-preparing-boot-media.html says

Use the dd utility to write the image. _Make sure you have the right drive!_
  # dd if=/path/to/Fedora-Live-Security-x86_64-21.iso of=/dev/sdd

as the '#' prompt hints, you need to be root tin order to run this command. It would be better to be explicit, saying something like

Use the dd utility to write the image onto the USB drive. You need to be root to do this, hence the example uses `sudo`. _Make sure you have the right drive!_
  $ sudo dd if=/path/to/Fedora-Live-Security-x86_64-21.iso of=/dev/sdd