Xamarin forms with .netStandard 2.0, can't reference Core

I can't reference my Interfaces from my .Core project to my Android Project and I don't understand why...

I created a new solution with Visual Studio 2017 in Xamarin.Forms with .Net Standard lib.

And this is what I tried:
I can't post link, so, delete space:
ht tp://us.v-cdn.net/5019960/uploads/editor/lx/58oajif2knff.png

With the old .NetFramework Core project method, it worked perfecly, but not now...

Thanks for your help