Error:Cause: unable to find valid certification path to requested target

This problem might be due to some restricted internet connections... If you are installing build tools on some restricted network, the required certificates might not install... So try changing the network connection.... Worked for me.....

I faced the same issue, but fixed it by adding

maven { url "http://jcenter.bintray.com"}

in the project Level Gradle File

buildscript {
repositories {
    jcenter()
    maven { url "http://jcenter.bintray.com"}
}
dependencies {
    classpath 'com.android.tools.build:gradle:2.2.3'
    classpath 'com.google.gms:google-services:3.0.0'
}
}
allprojects {
repositories {
    mavenCentral()
    jcenter{ url "http://jcenter.bintray.com/" }
    maven { url "https://jitpack.io" }
}
}
task clean(type: Delete) {
delete rootProject.buildDir
}