1.use lsblk to show which device is your usb device, make sure you have found the right device.

$ lsblk
sda           8:0    1  29.1G  0 disk 
└─sda1        8:1    1  29.1G  0 part /media/mdl/U
nvme0n1     259:0    0 238.5G  0 disk 
├─nvme0n1p1 259:1    0    28G  0 part /
├─nvme0n1p2 259:2    0     1K  0 part 
├─nvme0n1p5 259:3    0  15.7G  0 part [SWAP]
└─nvme0n1p6 259:4    0 194.9G  0 part /home

2.if your usb is mounted, just umount it

sudo umount /dev/sdX

3.use dd to write image to your usb device

sudo dd if=/path/to/ubuntu.iso of=/dev/sdX bs=4M && sync