Unlike modern versions where you simply sign in with a Trimble ID, the 2016 version required these manual entries to transition from "Trial Mode" to "Pro Mode." Where to Find Your Original License
If you still have SketchUp 2016 installed on an old computer, go to Help > License > License Info to view your serial number. sketchup 2016 license key and authorization code
This was the last "Free" desktop version. While also unsupported, it is often more stable on modern hardware than the 2016 version. Unlike modern versions where you simply sign in
Trimble provides a legacy "License Lookup" tool on their website. By entering the email address used at the time of purchase, you can have your old credentials resent to you. Can You Buy a SketchUp 2016 License Today? sketchup 2016 license key and authorization code