Here is my solution for key phrases of the messages: “unable to find valid certification path” and “PKIX path building failed“.
These phrases are shown when a SSL certification file can not be found.
Open https://jcenter.bintray.com/ in your browse and export the certificate in cer format.
Put the certificate in Android Studio’s cert folder
{your-home-directory}/.AndroidStudio3.0/system/tasks/cacerts
Then all went well.