The type or namespace name 'Page' could not be found (are you missing a using directive or an assemb

I am new here. I am trying mono out. I made a new application and I cannot build it.

The error is:
The type or namespace name 'Page' could not be found (are you missing a using directive or an assembly reference?)

I added Xamarin.Forms.Addin as a reference but it didnt do anything.

Any suggestions?

Thanks!