Cannot mount USB with anything, including Disk Utility
I'm guessing the drive is in a filesystem that is either not supported by Mac or corrupted. First, you should connect the drive, open Terminal and type

diskutil list


Then you'll see a few disks (shown similar to /dev/disk1) and their name and size. Based on their name/size, locate which one is the problematic drive and edit your question to include it's entry.

But either way, the case is probably that the drive is just not formatted so good. Mac can be really picky sometimes when it comes to disks, so I really recommend you try to burn the ISO onto the drive using Rufus. Rufus is an incredible tool which is also completely free, but unfortunately it's only available for Windows. So try to get yourself one (or maybe ask a friend to allow you to use his) so you can use this tool, then just open Rufus, choose your disk in the drop-down menu (make sure you choose the right disk!!), check the box saying "Create bootable disk" and click the disk button, choose you ubuntu ISO, and hit format. I use Rufus a lot and it hasn't failed me yet.