Problem generating Signed APK in android studio

I am trying to generate the Signed APK in android studio but it sends me the following error every time I try:

Please correct the above warnings first.

My build.gradle has the following code:

 android {
compileSdkVersion 26
defaultConfig {
    applicationId "edu.paquete.ejemplo"
    minSdkVersion 15
    targetSdkVersion 26
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
    release {
        minifyEnabled true
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}
 }

  dependencies {
implementation 'com.android.support:cardview-v7:26.1.0'
implementation 'Codeview-1.0.0:webviewcode:1.0.0'
implementation 'com.google.firebase:firebase-core:11.8.0'
implementation 'com.google.firebase:firebase-auth:11.8.0'
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'com.android.support:appcompat-v7:26.1.0'
implementation 'com.android.support:design:26.1.0'
implementation 'com.android.support.constraint:constraint-layout:1.1.0'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.1'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
 }
 apply plugin: 'com.google.gms.google-services'

At the moment of looking at what the error is, it is this that shows me:

 Java compiler (109 warnings)