I seem to be stuck in something that everyone finds easy. I cannot understand what is wrong with the manifest file or whatever is raising that error. I am a bit reluctant to change launch configurations because I believe that it could be an error that I have not seen. 
Here is a snippet from my manifest:

```
<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"
        android:label="@string/app_name"
        android:icon="@drawable/icon"
        android:theme="@style/AppTheme.NoActionBar">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
    </activity>
    <activity
        android:name=".ActivitySignUp"
        android:icon="@drawable/account"
        android:label="@string/sign_up">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
    </activity>
    <activity
        android:name=".ActivitySplash"
        android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity
        android:name=".ActivityDiscounts"
        android:icon="@drawable/discount"
        android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
    </activity>
</application>

```

I have tried these tricks severally:

1. Invalidate/Restart
2. Clean Project/Rebuild Project
3. Using a different android manifest file from a few hours ago
4. Refactored the relevant classes.

In addition to that, I have got "Unresolved class" warnings in all the activities as shown
[enter image description here][1][enter image description here][2]which makes me to believe it is an IDE issue since the same has manifested in all the projects including new ones that I have created to confirm that problem.