Xamarin XLS0502: The type 'Thickness' does not support direct content

You are trying to define a resource, and you could write this way as well:

<Thickness x:Key="PageMargin" Left="20" Top="20" Right="20" Bottom="20"/>

    <Thickness x:Key="PageMargin"  >
        <Thickness.Bottom>20</Thickness.Bottom>
    </Thickness>