android espresso do not find view

I am new to Android Espresso. When I try this

onView(withId(R.id.my_linear_layout)).check(matches(isDisplayed()));

I get this message

android.support.test.espresso.NoMatchingViewException: No views in hierarchy found matching: with id: com.echopen.asso.echopen:id/my_linear_layout

In the main activity's layout, here's how this linear layout appears

<RelativeLayout xmlns:android="http://ift.tt/nIICcg"
    android:id="@+id/main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >
<LinearLayout
        android:id="@+id/my_linear_layout"

but this layout do exist. How can it be ?