React-Native: Error type 3: Activity class does not exist

I'm aware there's a plethora of questions/answers regarding this issue. However, I have spent the past few days trying a number of solutions I've found on SO and none have worked thus far.

My error:

BUILD SUCCESSFUL in 52s
372 actionable tasks: 352 executed, 20 up-to-date
info Running C:\Users\User\AppData\Local\Android\Sdk/platform-tools/adb -s emulator-5554 reverse tcp:8081 tcp:8081
info Starting the app on emulator-5554 (C:\Users\User\AppData\Local\Android\Sdk/platform-tools/adb -s emulator-5554 shell am start -n mypackage/mypackage.MainActivity)...
Starting: Intent { cmp=mypackage/.MainActivity }
Error type 3
Error: Activity class {mypackage/mypackage.MainActivity} does not exist.

Here is a list of all the things I've attempted to do based on other SO posts:

Attempts

1. Running adb kill-server, adb start-server, then react-native run-android 和国内的大多数教程不同，这点非常起作用


2. Running adb reverse tcp:8081 tcp:8081, then react-native run-android (a senior dev I used to work with told me the issue might be related to this a long time ago, but I cannot get in touch with him)

3. Uninstalling the App from the Emulator, as well as my physical device

4. Build -> Clean Project

5. Files -> Invalidate Cache / Restart

6. Files -> Sync Project with Gradle Files

7. Build -> Rebuild Project

8. Deleted gradle cache files in /project/android/.gradle/5.1.1

9. AndroidManifest.xml -> <activity android:name=".MainActivity" to <activity android:name="packagename.MainActivity"

10. android/app/build.gradle to view buildTypes

debug {
  applicationIdSuffix ".debug"
}
stagingRelease {
  applicationIdSuffix ".staging"
}
release {

}

Ran react-native run-android --variant=[debug] --appIdSuffix '[.debug]'

&&

Ran react-native run-android --variant=[stagingRelease] --appIdSuffix '[.staging]'

(these came with different errors)

11. Make sure android/app/build.gradle

defaultConfig {
        applicationId "WHATEVER_THIS_IS" 
}

needs to match android/app/src/main/java/com/app_name/MainActivity.java line one:

package WHATEVER_THIS_IS;

needs to match android/app/src/main/java/com/app_name/MainApplication.java line one:

package WHATEVER_THIS_IS;

needs to match android/app/src/main/AndroidManifest.xml line one:

package="WHATEVER_THIS_IS">

12. Went to android/app/build.gradle and changed:

splits {
    abi {
        reset()
        enable enableSeparateBuildPerCPUArchitecture
        universalApk false  // If true, also generate a universal APK
        include "armeabi-v7a", "x86", "arm64-v8a", "x86_64"
    }
}

to

splits {
    abi {
        reset()
        enable true
        universalApk true  // If true, also generate a universal APK
        include "armeabi-v7a", "x86", "arm64-v8a", "x86_64"
    }
}

13. Went to C:/Users/User/.gradle to delete the cache. Was unable to delete everything, as I kept getting alerts that many of the files/folders inside are currently "open in another program" (untrue as far as I can tell)

Nothing worked.