You are probably not doing anything wrong.  It seems to me that the real issue is that the instruction, as written, don't work generically.  The point at which you are apparently stuck is the `chown` command, but all that is intended to accomplish is to make sure that "you" (really the `pi` user) actually owns the folder to be shared.  

If you created the `/home/pi/HDD` directory while logged in as the `pi` user, and *before* mounting the external drive, that's already done, so you can simply skip that step. As already mentioned in the answer you got, the FAT file system doesn't support such permissions, so they can't be applied after the drive is mounted.