Rufus is a Windows tool, so I assume that you try from Windows. It is usually working well, but if you have problems, you can try with Win32 Disk Imager.
If you are trying to use Rufus via Wine in Ubuntu, I suggest that you try a native linux tool, for example mkusb.
If there are still problems, there might be something wrong with the flash drive. Try to format it again in Windows, or to wipe and restore it to a storage drive with an MSDOS partition table and a FAT32 partition with mkusb in linux.
It this does not work, the drive is probably damaged in a way, that it difficult or impossible to repair with the tools we have available.