Missing a temporary folder

As I said in that thread, it’s a server configuration issue.

You see, whenever you upload something to a server through a web form, Apache has to have an empty folder where it can store the items while it’s processing them. If Apache doesn’t have that folder and can’t create it, it’s not going to allow uploads.

If you have root access to your server (and the requisite skills and knowledge) you could fix this yourself. Chances are you don’t have root access to your server (unless you’re on a dedicated server), so it’s easiest to ask your webhost to resolve this for you.