reuse Microsoft Office USB stick with 7.9 GB - Error: -69877: Couldn't open device - dd: /dev/disk9: Permission denied
After doing a little research on this, my conclusions are that you can try:


Restarting your machine. If you haven't restarted your machine since plugging in the USB drive then I would try that. Once you do, try reformatting the USB.
Running as root. In Terminal type in sudo su, press enter and then type in your password. Then issue the commands, being careful to type them in exactly as you need.
Booting into Safe mode. Another method is using Safe Mode, as this should allow you to encounter fewer errors than using terminal as a regular user. To boot into Safe Mode, immediately press and hold the Shift key down after booting up - keep it down until you see the login screen.


Finally, did you ever interrupt the formatting process at any time? Doing that can pose issues, so booting into Safe Mode is a good option.

Resources:  


https://origin-discussions-us.apple.com/thread/4806895?start=0&amp;tstart=0
https://apple.stackexchange.com/a/225061/242879  
https://askubuntu.com/a/771326