This is a separate, shorter code (often 8 digits) that authorizes the license key to work specifically on your machine.
You cannot use a 2018 or 2020 license key for the 2019 version. The key must match the software version. Moving Beyond 2019: The Shift to Subscriptions Sketchup Pro 2019 License Key And Authorization Number
When you first bought the software, Trimble (or your local reseller) sent an email with the subject line "SketchUp Pro License Information." Search your inbox for "SketchUp" or "SketchUp License." This is a separate, shorter code (often 8