The Blizzard Authenticator is incredibly difficult to set up.
*Clicked on my WoW bag.
*Interred in the code sent to my email.
*Downloaded the App on my phone(after clicking on mobile authenticator).
*Entered log in info on authenticator phone app…Stuck on “setting up”
Hackers are running unchecked! Please help!
I’m not what this “WoW bag” is referring to, given the fact you don’t need to do anything in game to set one up. Along with that, I’m not sure what hackers have to do with this.
Side note: Want to change the title as cursing, even if it’s shorthand, is against the rules.
I think that when you don’t have an authenticator attached, the “extra bag slots” are promoted to you when you open your bag. Clicking on that probably opens up the web page for getting an authenticator set up.
This is the only issue. @OP: I’d write down every code and serial number you’ve been given thus far, and if you are still stuck on “setting up,” (40 minutes has elapsed since the original post) then reboot your phone and try again.
This isn’t a difficult process at all, it’s actually part and parcel of installing programs (ecch, I’m showing my age, I am sorry … “apps”) on a phone or computer; sometimes the installation hangs, and you need to try again.
Good luck! 
I know this is a months old discussion now but I was having this same problem recently and followed all the directions on the support page and still couldn’t get it to work. Until I read here that in order to receive the backpack upgrade (which is why I downloaded the Authenticator in the first place) you need to also enable phone notifications. So I did that, then uninstalled/reinstalled the Authenticator app and, lo and behold, instead of getting stuck again, it sent a code to my phone which I typed into the app and, voilà! It gave me a serial number and restore code and I was good to go! 
So try adding phone notifications and see if that fixes your problem. 