In the output of ls -l /dev/sda there should be brw-rw---- os so, where b denotes a block device file. In your case it's -rw-r--r-- where the first - denotes a regular file. This is wrong.
The easiest explanation is /dev/sda did not exist when you run your dd command and the command created it as a regular file.
/dev/sda may have not existed because you had removed it or because your flash drive was (and maybe still is) something else, e.g. /dev/sdb. (I don't know Chrome OS; in Debian/Ubuntu the right tool to find out is lsblk.)
You got No space left on device because the regular file grew inside your /dev which is a virtual filesystem in memory and has a low size limit. It is only meant to contain device files, directories and symlinks, not regular files.
Remove the regular file: sudo rm /dev/sda. Disconnect the flash drive and connect it again. Find out the right device file (sudo dmesg or lsblk may be handy). Confirm that in the output of ls -l /dev/right_device_file there is b just before rw…. Then proceed with dd, this time using the right device file.