First have a look at the partitions and file systems on the system with this command:
sudo fdisk -l
The drive that you’d like to target might be something like /dev/sdc, /dev/sdd or /dev/sde. You can get the target drive from the fdisk -l command.
Then locate the path of the iso image. Assume that the path of iso image is: /home/user/Downloads/ubuntu-18.04-desktop-amd64.iso
and the path of target drive is: /dev/sdc
Then command to create the bootable USB flash drive is
sudo dd if=/home/user/Downloads/ubuntu-18.04-desktop-amd64.iso of=/dev/sdc status=progress
status=progress will give you the progress report on the terminal.