If you own a legitimate 2017 Pro license but are getting errors like "Invalid Serial Number" or "Authorization Failed," try these steps: SketchUp 2017 Checkup
As of November 4, 2020, Trimble (the parent company) discontinued the "Classic" perpetual license. This means:
If you bought a license years ago but never "deauthorized" it from an old computer, you may find the system blocks you from activating it on a new one.