This is the error I received:
(env) $ heroku addons:create mongolab:sandbox
Creating mongolab:sandbox on ⬢ playlister-zr... !
▸ Couldn't find either the add-on service or the add-on plan of "mongolab:sandbox".
I think we'll need to update the section on "Adding a Production Database" sometime in the future. This blog looks like it may be able to help with this (I haven't read it too deeply yet).
This doesn't prevent me from pushing my code to Heroku, but it will prevent someone from actually using the deployed website.