title: How to fix this > attribute app:constraintBottom_toBottomOf not found
body: <p>Android newbie here. </p>
  
  <p>I am building and AR app and now I'm planning to add a User Interface. I am trying to add a constraint layout to <code>app:layout_constraintBottom_toBottomOf="parent"</code>, however Im getting the error that this constraint was not found</p>
  
  <p>These are my codes and error messages:</p>
  
  <p><img src="https://i.stack.imgur.com/mjLJf.jpg" alt="code and error">
  <img src="https://i.stack.imgur.com/cwQuv.jpg" alt="more of error"></p>
  
  <p>All the lines of code I add in my activity_ux.xml file that begin with "app"
  e.g. <code>app:layout_constraintBottom_toTopOf</code> do not seem to work, I just showed you an example here with one of them just to make it easier to debug I think.</p>
  
  <p>I tried searching online, one of the solutions was that I should install "ConstraintLayout for Android" and "Solver for ConstraintLayout" in my SDK Tools, but I already had this installed.</p>
  
  <pre><code>    &lt;FrameLayout 
            xmlns:android="http://schemas.android.com/apk/res/android"
            xmlns:tools="http://schemas.android.com/tools"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            xmlns:app="http://schemas.android.com/apk/res-auto"
            tools:context="com.google.ar.sceneform.Overlei.Overlei"&gt;
  
      &lt;fragment android:name="com.google.ar.sceneform.ux.ArFragment"
            android:id="@+id/ux_fragment"
            android:layout_width="match_parent"
            android:layout_height="match_parent" /&gt;
  
      &lt;LinearLayout
            android:id="@+id/gallery_layout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:orientation="horizontal"
            app:layout_constraintBottom_toBottomOf="parent"
    &gt;
  
     &lt;/LinearLayout&gt;
  
     &lt;/FrameLayout&gt;
  </code></pre>
  
  <p>I expected this to just work, I don't know where I can start looking if an in-built function(such as this) does not work.</p>