سلام وقت بخیر

شما نام فیلد رو تغییر دادین پس باید طبق اون برنامه تون رو جلو ببرید 

@ColumnInfo(name = "completed")
 private boolean isCompleted;

پس متد add دیتابیس شما میشه(مشکل تون این بود که بجای completed بنویسید isCompleted رو نوشتید .

public long addTask(Task task){
        SQLiteDatabase sqLiteDatabase = getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put("title" , task.getTitle());
        contentValues.put("completed",task.isSelected()); ***
        long result = sqLiteDatabase.insert(TABLE_TASKS,null , contentValues);
        sqLiteDatabase.close();
        return result;
    }

و بطور کل کلاس شما میشه:

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
import androidx.annotation.Nullable;
import java.util.ArrayList;
import java.util.List;
public class SQLiteHelper extends SQLiteOpenHelper {
    private static final String TABLE_TASKS = "tbl_tasks";
    public SQLiteHelper(@Nullable Context context) {
        super(context, "db_app", null, 1);
    }
    @Override
    public void onCreate(SQLiteDatabase db) {
        try {
            db.execSQL("CREATE TABLE "+ TABLE_TASKS + " (id INTEGER PRIMARY KEY AUTOINCREMENT, title TEXT, completed BOOLEAN);");
        }catch (SQLiteException e){
            Log.e("TAG", "onCreate: "+e.toString() );
        }
    }
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    }
    public long addTask(Task task){
        SQLiteDatabase sqLiteDatabase = getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put("title" , task.getTitle());
        contentValues.put("completed",task.isSelected());
        long result = sqLiteDatabase.insert(TABLE_TASKS,null , contentValues);
        sqLiteDatabase.close();
        return result;
    }
    public List<Task> getTasks(){
        SQLiteDatabase sqLiteDatabase = getReadableDatabase();
        Cursor cursor = sqLiteDatabase.rawQuery("SELECT * FROM "+TABLE_TASKS ,null);
        List<Task> tasks = new ArrayList<>();
        if(cursor.moveToFirst()){
            do{
                Task task = new Task();
                task.setId(cursor.getLong(0));
                task.setTitle(cursor.getString(1));
                task.setSelected(cursor.getInt(2)==1);
                tasks.add(task);
            }while (cursor.moveToNext());
        }
        return tasks;
    }
    public int updateTask(Task task){
        SQLiteDatabase sqLiteDatabase = getWritableDatabase();
        ContentValues contentValues = new ContentValues();
        contentValues.put("title" , task.getTitle());
        contentValues.put("completed",task.isSelected());
        int result = sqLiteDatabase.update(TABLE_TASKS , contentValues , "id = ?" , new String[]{String.valueOf(task.getId())});
        sqLiteDatabase.close();
        return result;
    }
    public int deleteTask(Task task){
        SQLiteDatabase sqLiteDatabase = getWritableDatabase();
        int result = sqLiteDatabase.delete(TABLE_TASKS , "id = ?" ,new String[]{String.valueOf(task.getId())});
        sqLiteDatabase.close();
        return result;
    }
    public void ClearAllTasks(){
        SQLiteDatabase sqLiteDatabase = getWritableDatabase();
        sqLiteDatabase.execSQL("DELETE FROM "+TABLE_TASKS);
    }
    public List<Task> searchInTasks(String query){
        SQLiteDatabase sqLiteDatabase = getWritableDatabase();
        Cursor cursor = sqLiteDatabase.rawQuery("SELECT * FROM "+TABLE_TASKS+" WHERE title LIKE '%"+query+"%'",null);
        List<Task> tasks = new ArrayList<>();
        if(cursor.moveToFirst()){
            do{
                Task task = new Task();
                task.setId(cursor.getLong(0));
                task.setTitle(cursor.getString(1));
                task.setSelected(cursor.getInt(2)==1);
                tasks.add(task);
            }while (cursor.moveToNext());
        }
        return tasks;
    }
}

بخاطر اینکه سورس پروژه جزیی از دوره هست من لینک دانلود پروژه رو حذف کردم 

ممنونم از شما.