Google said:Use of Google Earth Pro requires a valid license key. As of January 20, 2015, Earth Pro licenses are now free. After getting your Earth Pro license, you can register, download, install, and sign in to start using Earth Pro's advanced features.
If you do not have a key, use your email address and the key GEPFREE to sign in.
https://support.google.com/earth/answer/176160?hl=en