Unlike modern versions where you simply sign in with a Trimble ID, the 2016 version validates your credentials against a license server. The Reality of Legacy Support
SketchUp has moved to a subscription model. If you need the latest Pro features, checking their current plans is the only way to get a guaranteed working license [1, 2]. Do you still have the email address Sketchup Pro 2016 License Key And Authorization Number
If you are a legal owner of a SketchUp Pro 2016 license and simply lost your numbers, you don't need to look for "cracks" or keygens. You can retrieve your original info through official channels: Unlike modern versions where you simply sign in