This is not a mistake, but a warning. And it was not aimed at you, but the proponents of the package have some package that you download from the starter kit.
The fact is that vanilla is Emacs 24.3, which does not give warnings, and then there are various libraries written for earlier versions of Emacs that you use for Emacs 24.3. Hence the warning.
It will still work, but probably when the next Emacs package comes out, the assoc package will generate an error instead of a warning, as it is now.
Now you can choose whether you want emacs-starter-kit or vanilla Emacs or something else. I learned my way from Vanilla Emacs by customizing myself.
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