Hi @Arthur_Cheung ,
you do know that the instructions you were following are for RaspberryPi don't you?, I ask because you posted your help request in the normal help & support section.
If you want a normal Ubuntu installation USB stick then please take a look here:
If unetbootin doesn't work for you, try "mintstick". You should (can) also install udisks2 in Ubuntu 15.10 and higher!:
http://pkgs.org/search/mintstick
http://pkgs.org/download/udisks2
Firstly, download your preferred version of Ubuntu Mate OS, 32 or 64 Bit (if you are unsure, download the 32 Bit version) (Ubuntu/Debian/Voyager/PinguyOS etc) in the form of an ISO file, remember where you store the file!. (I am not talking about using the Unetbootin download function, I am talking about you d…
To clean the USB stick first; use Gparted:
PLEASE NOTE: THIS GUIDE DOES NOT DEPICT THE INSTALLATION (CREATION) OF A UEFI PARTITION. IF YOU HAVE A COMPUTER WITH UEFI/BIOS, MAKE SURE YOU ALSO CREATE A UEFI/BIOS PARTITION AS WELL. SEE FURTHER REMARKS ABOUT UEFI PARTITIONS IN THE GUIDE!.
Linux beginners should also look here:
You can only partition on an unmounted drive using an installation disk of your Linux flavour with a partitioning tool included, use the Gparted tool in live mode, or you can download a copy of Gparted from the link…