Android Studio – Unresolved tag Android Studio

I’am working on an app that can download, modify, read, show and share PDF. To do this, I found a youtube video that use some external libraries on github. I tried to add to my project (especially in an xml file) a PDF Viewer from a repository that the video advised to use but when I run into a problem and I don’t Know why. The program says that there is an "unresolvable tag" but I don’t Know how to resolve it. I attach the full code below, thanks for help.

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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">

<TextView android:id="@+id/tcTitle"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="PDF Viewer Demo"
    android:textStyle="bold"
    android:textSize="18sp"
    android:layout_margin="10dp"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    />

<com.github.barteksc.pdfviewer.PDFView
    android:id="@+id/pdfView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/tcTitle"
    android:background="@color/white"
    android:layout_marginTop="10dp"
    android:layout_marginStart="10dp"
    android:layout_marginEnd="10dp"
    android:layout_marginBottom="70dp"
    />

<Button android:id="@+id/shareButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintBottom_toBottomOf="parent"
    android:text="SHARE PDF"
    android:padding="10dp"
    android:layout_margin="20dp"
    />

<ProgressBar android:id="@+id/progressBar"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    app:layout_constraintBottom_toBottomOf="parent"
    />
    </androidx.constraintlayout.widget.ConstraintLayout>