Problems with http://schemas.android.com/apk/res/android

Currently I'm getting into the Android programming with Visual Studio v3 and Xamarín v4.3 .

The project that I am creating is to make a layout of login, but I have several problems with http://schemas.android.com/apk/res/android , because in some lines of the XML , it tells me that the attributes have not been defined.

How can I solve this problem in order to understand it and, in the future, be able to solve it with the other projects that I will create?

Code:

<RelativeLayout>
    //...
    <ProgressBar
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true" //El error es aquí.
        //...
    />
</RelativeLayout>

Error:

  

The http://schemas.android.com/apk/res/android:layout_centerInParent attribute was not declared.