Error 1

Error Log:

A problem occurred configuring root project 'app'.
 > Could not resolve all files for configuration ':classpath'.
    > Could not download kotlin-reflect.jar (org.jetbrains.kotlin:kotlin-reflect:1.1.3-2)
       > Could not get resource 'https://jcenter.bintray.com/org/jetbrains/kotlin/kotlin-reflect/1.1.3-2/kotlin-reflect-1.1.3-2.jar'.
          > Could not GET 'https://jcenter.bintray.com/org/jetbrains/kotlin/kotlin-reflect/1.1.3-2/kotlin-reflect-1.1.3-2.jar'.
             > Software caused connection abort: recv failed
> Could not download protobuf-java.jar (com.google.protobuf:protobuf-java:3.0.0)
   > Could not get resource 'https://jcenter.bintray.com/com/google/protobuf/protobuf-java/3.0.0/protobuf-java-3.0.0.jar'.
      > Could not GET 'https://jcenter.bintray.com/com/google/protobuf/protobuf-java/3.0.0/protobuf-java-3.0.0.jar'.
         > Software caused connection abort: recv failed

Solution:
modify config file UE_4.20\Engine\Build\Android\Java\gradle\build.gradle,
add these into build.gradle

jcenter() {
        url "http://jcenter.bintray.com/"
    }

all content:

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {
        google()
        jcenter() {
            url "http://jcenter.bintray.com/"
        }
    }
    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
    }
    apply from: 'buildscriptAdditions.gradle', to: buildscript
}

apply from: 'baseBuildAdditions.gradle'

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

task clean(type: Delete) {
    delete rootProject.buildDir
}
Error 2

Error Log:

A problem occurred configuring root project 'app'.
 > Could not resolve all files for configuration ':classpath'.
    > Could not download kotlin-reflect.jar (org.jetbrains.kotlin:kotlin-reflect:1.1.3-2)
       > Could not get resource 'http://jcenter.bintray.com/org/jetbrains/kotlin/kotlin-reflect/1.1.3-2/kotlin-reflect-1.1.3-2.jar'.
          > Could not GET 'http://jcenter.bintray.com/org/jetbrains/kotlin/kotlin-reflect/1.1.3-2/kotlin-reflect-1.1.3-2.jar'.
             > Remote host closed connection during handshake
    > Could not download kotlin-stdlib.jar (org.jetbrains.kotlin:kotlin-stdlib:1.1.3-2)
       > Could not get resource 'http://jcenter.bintray.com/org/jetbrains/kotlin/kotlin-stdlib/1.1.3-2/kotlin-stdlib-1.1.3-2.jar'.
          > Could not HEAD 'http://jcenter.bintray.com/org/jetbrains/kotlin/kotlin-stdlib/1.1.3-2/kotlin-stdlib-1.1.3-2.jar'.
             > Remote host closed connection during handshake
    > Could not download protobuf-java.jar (com.google.protobuf:protobuf-java:3.0.0)
       > Could not get resource 'http://jcenter.bintray.com/com/google/protobuf/protobuf-java/3.0.0/protobuf-java-3.0.0.jar'.
          > Could not GET 'http://jcenter.bintray.com/com/google/protobuf/protobuf-java/3.0.0/protobuf-java-3.0.0.jar'.
             > Remote host closed connection during handshake
    > Could not download bcpkix-jdk15on.jar (org.bouncycastle:bcpkix-jdk15on:1.56)
       > Could not get resource 'http://jcenter.bintray.com/org/bouncycastle/bcpkix-jdk15on/1.56/bcpkix-jdk15on-1.56.jar'.
          > Could not HEAD 'http://jcenter.bintray.com/org/bouncycastle/bcpkix-jdk15on/1.56/bcpkix-jdk15on-1.56.jar'.
             > Remote host closed connection during handshake
    > Could not download bcprov-jdk15on.jar (org.bouncycastle:bcprov-jdk15on:1.56)
       > Could not get resource 'http://jcenter.bintray.com/org/bouncycastle/bcprov-jdk15on/1.56/bcprov-jdk15on-1.56.jar'.
          > Could not HEAD 'http://jcenter.bintray.com/org/bouncycastle/bcprov-jdk15on/1.56/bcprov-jdk15on-1.56.jar'.
             > Remote host closed connection during handshake
    > Could not download fastutil.jar (it.unimi.dsi:fastutil:7.2.0)
       > Could not get resource 'http://jcenter.bintray.com/it/unimi/dsi/fastutil/7.2.0/fastutil-7.2.0.jar'.
          > Could not HEAD 'http://jcenter.bintray.com/it/unimi/dsi/fastutil/7.2.0/fastutil-7.2.0.jar'.
             > Remote host closed connection during handshake
 * Try:
 Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
 * Get more help at https://help.gradle.org

Solution:
Project Settings -> Platforms -> Android -> Uncheck “Enable Gradle instead of Ant”