Unresolved class 'MyFirebaseMessagingService' Android Studio

Nikola Arsovski :

I have problem with Firebase in Android studio. Here is what happen: In debug mod everything is working fine with follow AndroidManifest

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.hajora.dndcommerce"
android:installLocation="auto"
android:versionCode="2"
android:versionName="1.0"
xmlns:tools="http://schemas.android.com/tools">
<uses-sdk
    android:minSdkVersion="11"
    android:targetSdkVersion="21"
    tools:overrideLibrary="com.google.firebase.messaging"
    />
<supports-screens
    android:largeScreens="true"
    android:normalScreens="true"
    android:smallScreens="true"
    android:xlargeScreens="true"
    android:anyDensity="true"
 />
<uses-permission android:name="android.permission.CALL_PHONE"  />
<uses-feature android:name="android.hardware.telephony" android:required="false" />
<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

<application
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:theme="@style/AppTheme" >
    <activity
        android:name="com.hajora.dndcommerce.Education"
        android:label="@string/app_name">
    </activity>
    <activity android:name="com.hajora.dndcommerce.Splashscreen">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <action android:name="com.google.firebase.MESSAGING_EVENT"/>
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name="com.hajora.dndcommerce.SingleItemViewEducation" />
    <activity android:name="com.hajora.dndcommerce.SingleItemViewNews" />
    <activity android:name="com.hajora.dndcommerce.Main" />
    <activity android:name="com.hajora.dndcommerce.Submit" />
    <activity android:name="com.hajora.dndcommerce.News" />
    <activity android:name="com.hajora.dndcommerce.Contact" />
    <activity android:name="com.hajora.dndcommerce.About" />
    <activity android:name="com.hajora.dndcommerce.AboutCompany" />
    <activity android:name="com.hajora.dndcommerce.AboutHistory" />
    <activity android:name="com.hajora.dndcommerce.AboutVision" />
    <activity android:name="com.hajora.dndcommerce.AboutPartners" />
    <activity android:name="com.hajora.dndcommerce.Ask"
        android:windowSoftInputMode="stateUnchanged" />
    <activity android:name=".Partner" />
</application>
    <service
        android:name=".MyFirebaseMessagingService">
        <intent-filter>
            <action android:name="com.google.firebase.MESSAGING_EVENT"/>
        </intent-filter>
    </service>
    <service
        android:name=".MyFirebaseInstanceIDService">
        <intent-filter>
            <action android:name="com.google.firebase.INSTANCE_ID_EVENT"/>
        </intent-filter>
    </service>

When i switch to release mode, i get the error:

Error:(54) Error: The <service> element must be a direct child of the <application> element [WrongManifestParent]

So, i then put both services inside tag and after that i get the error:

Unresolved class 'MyFirebaseMessagingService' less... (Ctrl+F1)

Validates resource references inside Android XML files.

I cannot figure what is the problem

UPDATE gradle

    apply plugin: 'com.android.application'

android {

    compileSdkVersion 21
    buildToolsVersion "26.0.0"
    defaultConfig {
        applicationId 'com.hajora.dndcommerce'
        minSdkVersion 21
        targetSdkVersion 21
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
            signingConfig signingConfigs.release
        }
    }
    productFlavors {
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_7
        targetCompatibility JavaVersion.VERSION_1_7
    }
}

dependencies {
    compile files('libs/activation.jar')
    compile files('libs/mail.jar')
    compile files('libs/additionnal.jar')
    compile 'com.google.firebase:firebase-messaging:10.0.1'
    compile 'com.android.support.constraint:constraint-layout:1.0.2'
    compile 'com.google.firebase:firebase-core:10.0.1'
}
apply plugin: 'com.google.gms.google-services'

Mohammed Farhan :

See this image first

check if Firebase classes are in correct package folder. example if firebase classes are in package 'services' then in manifest service would have android:name=".services.FirebaseMessagingService" then Unresolved class 'MyFirebaseMessagingService' error will get solved.