How to get rid of overlapping elements?

Question

XML

I apologize right away, I'm just getting started ....
Task : 4 buttons vertically - one under one through a LinearLayout.
The code

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <LinearLayout
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:background="#212121"
        android:gravity="center_vertical"
        android:orientation="vertical"
        android:visibility="visible">

        <android.support.percent.PercentRelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:app="http://schemas.android.com/apk/res-auto"
            android:layout_width="fill_parent"
            android:layout_height="fill_parent">

            <Button
                android:id="@+id/button2"
                android:background="#239461"
                android:text="1"
                android:textColor="#ffffff"
                android:visibility="visible"
                app:layout_heightPercent="20%"
                app:layout_marginRightPercent="2%"
                app:layout_widthPercent="75%"
                tools:ignore="MissingConstraints" />

            <Button
                android:id="@+id/button3"
                android:background="#239461"
                android:text="2"
                android:textColor="#ffffff"
                android:visibility="visible"
                app:layout_heightPercent="20%"
                app:layout_marginRightPercent="2%"
                app:layout_widthPercent="75%"
                tools:ignore="MissingConstraints" />

            <Button
                android:id="@+id/button4"
                android:background="#239461"
                android:text="3"
                android:textColor="#ffffff"
                android:visibility="visible"
                app:layout_heightPercent="20%"
                app:layout_marginRightPercent="2%"
                app:layout_widthPercent="75%"
                tools:ignore="MissingConstraints" />

            <Button
                android:id="@+id/button5"
                android:background="#239461"
                android:text="4"
                android:textColor="#ffffff"
                android:visibility="visible"
                app:layout_heightPercent="20%"
                app:layout_marginRightPercent="2%"
                app:layout_widthPercent="75%"
                tools:ignore="MissingConstraints" />
        </android.support.percent.PercentRelativeLayout>
    </LinearLayout>
</android.support.constraint.ConstraintLayout>

But instead it turns out that the buttons stick to the upper left corner and, accordingly, overlap:
How to fix?