android.widget.Toolbar cannot be converted to android.support.v7.widget.Toolbar

if your project is already migrated to AndroidX then import this
import androidx.appcompat.widget.Toolbar;

else migrate to AndroidX by
Refactor>Migrate to AndroidX Full code Example of XML and Java

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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"
android:orientation="vertical"
tools:context=".Events.Events_Main_Activity">
<androidx.coordinatorlayout.widget.CoordinatorLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content">
    <com.google.android.material.appbar.AppBarLayout
        android:id="@+id/appbarlayout_id"
        android:layout_width="match_parent"
        android:layout_height="300dp"
        android:theme="@style/ThemeOverlay.AppCompat.Dark.ActionBar">
        <com.google.android.material.appbar.CollapsingToolbarLayout
            android:id="@+id/collapsingtoolbar_id"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:contentScrim="?attr/colorPrimary"
            app:expandedTitleMarginEnd="8dp"
            app:expandedTitleMarginStart="8dp"
            app:layout_scrollFlags="exitUntilCollapsed|scroll"
            app:title="Anime Title"
            app:titleEnabled="true">
            <ImageView
                android:id="@+id/aa_thumbnail"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                android:background="@drawable/bg_gradient" />
            <androidx.appcompat.widget.Toolbar
                android:id="@+id/toolbar_sec"
                android:layout_width="match_parent"
                android:layout_height="?attr/actionBarSize"
                app:title="shshhs" />
        </com.google.android.material.appbar.CollapsingToolbarLayout>
    </com.google.android.material.appbar.AppBarLayout>
</androidx.coordinatorlayout.widget.CoordinatorLayout>
</LinearLayout>



package com.uafappdevelopers.example;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
//note this import
import androidx.appcompat.widget.Toolbar;
import com.uafappdevelopers.uafportal.R;
public class Events_Second_Activity extends AppCompatActivity {
//add this for the navigation back button click event
@Override
public boolean onSupportNavigateUp() {
    onBackPressed();
    finish();
    return super.onSupportNavigateUp();
}
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_event_second);
    //change id to Your id
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar_sec);
    setSupportActionBar(toolbar);
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
    getSupportActionBar().setDisplayShowHomeEnabled(true);
    setTitle("University of Agriculture Faisalabad");
}
}

You use android.support.v7.widget.Toolbar in your XML, but in your java code you are importing android.widget.Toolbar which is a different type. Change your import to android.support.v7.widget.Toolbar and it should work.