Error adding Design Library

Question

I am fresher in developing Android Apps. I need a tabbed layout and added Design library for tabbed layout in my Android project. I get several errors with a common description as below.

Unable to resolve dependency for ':app@releaseUnitTest/compileClasspath': 
Could not resolve com.android.support:design:26.1.0.

Could not resolve com.android.support:design:26.1.0.
Required by:
project :app
> Could not resolve com.android.support:design:26.1.0.
> Could not get resource 'https://dl.google.com/dl/android/maven2/com/android/support/design/26.1.0/design-26.1.0.pom'.
      > Could not GET 'https://dl.google.com/dl/android/maven2/com/android/support/design/26.1.0/design-26.1.0.pom'.
               > sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
                           > PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
                                          > unable to find valid certification path to requested target

Module:app

apply plugin: 'com.android.application'

android {
compileSdkVersion 26
defaultConfig {
    applicationId "com.naari.simpleApp"
    minSdkVersion 23
    targetSdkVersion 26
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}
}

dependencies {
implementation fileTree(include: ['*.jar'], dir: 'libs')
implementation 'com.android.support:appcompat-v7:26.1.0'
implementation 'com.android.support.constraint:constraint-layout:1.0.2'
implementation 'com.android.support:design:26.1.0'
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'

}

Project:AndroProject

buildscript {

repositories {
    google()
    jcenter()


}
dependencies {
    classpath 'com.android.tools.build:gradle:3.0.1'


    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files
}

}

allprojects {
repositories {
    google()
    jcenter()
}
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

What should I do to fix it? I went through this link, but it did not fix. I have no certificates under Settings-->Server certificates. Any thoughts?