You need Unity version 2017.2b9, b11, or f1. 2017.2b1 did not include support for ARCore. Also, you might get this error if you hit the Play in Editor button. ARCore only works on a supported Android device (Pixel or S8) and will throw a DllNotFoundException if you try to play in the Editor.
Looks like 2017.2f1 was never released; the closest would be 2017.2.0f3. 2017.2.0p1 should still work with ARCore, though this was the first of several patch releases and won’t be as stable.