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Manifestation merger failed with multiple errors, see logs

I've checked the merged manifest to find these errors
(Note that I'm a beginner android developer so every problem in android studio seems like the end of the world to me. Please help me solve this error, thanks in advance)
ERROR: tools:replace specified at line:6 for attribute android:appComponentFactory, but no new value specified App_Name.app main manifest (this file), line 5
ERROR: Validation failed, exiting App_Name.app main manifest (this file)
My Manifest.xml file
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
package="com.syddhoo.bookapp">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/Theme.BookApp"
tools:replace="android:appComponentFactory">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>

Every time when i open class teacher's project to my system it shows this error.Error running app: Default activity not found

I'm a beginner in Android.
Every time when I open class teacher's project to my system it shows this error.
Error running app: Default activity not found.
What should I do to match the same project as my teacher's?
I've tried many solutions.
My Manifest file is
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.assigncheckbox">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>

maybe it happen because you didn't add this activity to your AndroidManifest.xml
<activity
android:name=".yourActivity"
/>

MainActivity unresolved class error with android studio

Im taking the udacity coarse and for some reason android studio wont recognize my MainActivity class. Ive tried cleaning the project, rebuilding and making sure that the names were correct but nothing has solved my problem.
Android Manifest code:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="androidedx.example.activitylifecycle">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".Activity1"></activity>
<activity android:name=".Activity2" />
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>

Found the problem, this package name within the code I had to paste
package com.example.android.cookies;
Im not sure why specifically this caused the problem but atleast I solved it.

Could not identify launch Activity

have had this problem for a while I just installed android studio and I have been having this problem:
Could not identify launch activity: Default Activity not found Error while Launching activity
... I have this problem with existing projects MyApp I tried to make new project but the same error still
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.android.app.myapplication">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN"/>
<category android:name="android.intent.category.LAUNCHER"/>
</intent-filter>
</activity>
</application>
</manifest>

File -> Invalidate Caches / Restart...

Why cant i sync my project in Android Studio?

I wanted to learn how to make android apps and downloaded android studio.
But when I made my first project and it was trying to sync an error came. This is what it said: A problem occurred configuring project ':app'.
When i went to Android Manifest most of it was red. Does someone know what to do?
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.oliver.mycart">
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:abel="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity
android:name=".Menu"
android:label="#string/app_name"
android:theme="#style/AppTheme.NoActionBar">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>

android:abel="#string/app_name"
you have error in this line. Try to change abel to label