generate signed apk is not working, apk is unsigned

I used to generate a signed apk from Android Studio and everything was working well until I updated Android Studio to 3.3. It generates an apk but after I try to install it, it says: App Not Installed!

My Trial was by:

1. Click on build
2. Generate Signed Apk
3. Choose APK and click Next
4. Insert the Key Store Path, Key Store Password, Key Alias, Key Password
5. Click Next
6. Choose Release Variant
7. Click Finish

The Apk is generated but it's not signed! What is the problem ?

Here is my app build.gradle

buildscript {
    repositories {
        maven { url 'https://maven.fabric.io/public' }
    }

    dependencies {
        classpath 'io.fabric.tools:gradle:1.+'
    }
}
apply plugin: 'com.android.application'
apply plugin: 'io.fabric'

repositories {
    maven { url 'https://maven.fabric.io/public' }
}


android {
    useLibrary 'org.apache.http.legacy'
    compileSdkVersion 27
    defaultConfig {
        applicationId "com.twobreathe.soft2breathe"
        minSdkVersion 23
        targetSdkVersion 27
        versionCode 9
        versionName "1.1"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        externalNativeBuild {
            cmake {
                cppFlags ""
            }
        }
        vectorDrawables {
            useSupportLibrary true
        }
        resConfigs "en", "ja"
    }
    signingConfigs {
        release {
            keyAlias "[my key alias]"
            keyPassword "[my key password]"
            storeFile file("[path to the keystore file]")
            storePassword "[my store password]"
        }
    }

    buildTypes {
        release {
            minifyEnabled true
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
            shrinkResources true
            pseudoLocalesEnabled false
        }

        debug {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
            debuggable true
        }


    }
    externalNativeBuild {
        cmake {
            path "src/main/cpp/fluidsynth/android/CMakeLists.txt"
        }
    }
    lintOptions {
        checkReleaseBuilds false
        abortOnError false
    }
    dataBinding {
        enabled true
    }
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
    buildToolsVersion '28.0.3'
    productFlavors {
    }
}

dependencies {
    implementation fileTree(include: ['*.jar'], dir: 'libs')
    implementation 'com.android.support:appcompat-v7:27.1.1'
    implementation 'com.android.support.constraint:constraint-layout:1.1.3'
    implementation 'com.github.parse-community.Parse-SDK-Android:parse:1.18.4'
    implementation 'com.jjoe64:graphview:4.2.2'
    implementation 'io.reactivex.rxjava2:rxjava:2.2.0'
    implementation 'io.reactivex.rxjava2:rxandroid:2.1.0'
    testImplementation 'junit:junit:4.12'
    implementation 'com.orhanobut:hawk:2.0.1'
    implementation 'xyz.sahildave:arclayout:1.0.0'
    implementation 'com.mikhaellopez:circularprogressbar:2.0.0'
    androidTestImplementation 'com.android.support.test:runner:1.0.2'
    implementation 'com.github.GrenderG:Toasty:1.3.0'
    implementation 'com.kyleduo.switchbutton:library:2.0.0'
    implementation 'com.github.franmontiel:LocaleChanger:0.9.2'
    implementation 'cn.aigestudio.wheelpicker:WheelPicker:1.1.2'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
    implementation 'com.crashlytics.sdk.android:crashlytics:2.9.8'
}

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