"Could not write the disc image to the device" error when creating an Ubuntu startup USB

Question

I'm following the instructions on installing Ubuntu from USB:

1. I have downloaded the .ISO file.
2. Inserted a 16 GB USB.
3. Launched Startup Disk Creator from the Dash.
4. Followed prompts.

I then get this error:

Could not write the disc image (/home/clintonlucas/Downloads/ubuntu-16.04.3-desktop-amd64.iso) to the device (/dev/sdc).

No other info given.

Help please.