It can be tricky sometimes, but here's the general outline of what you should be doing:
Schedule a post on Buffergram (this one will go live once it works, keep that in mind!) and make it so that it'll post right away. It will take about 1-2 minutes for our system to pick up your post and send it to the machines. Now, this may work straight away and all is good.
If not (that's the more likely variant), our machine will fail verification. Instagram (and that's smart of them!) sees a new device trying to use your account and stops us right there. That's good.
Now, you take your device and log in to Instagram (log out and log in again!) — if everything works, you should now see a screen that informs you about this new device and asks you if that was you. You have to click "IT WAS ME", since our devices are authorized by you now.
And that's it, it should work now. Go back to Buffergram and re-schedule that same post. It will go through this time.
If not, drop us a note in the contact form below or to: firstname.lastname@example.org and we usually deal with it very quickly.