If the software is already installed and working on an old computer, go to Help > License (Windows) or SketchUp > License (Mac) to view your active serial number. Steps to Authorize SketchUp Pro 2017
This often happens if you try to use a 2017 license on a 2018 or newer version. Licenses are version-specific. Sketchup 2017 License Key And Authorization Number
Look for a message from sketchupmail@sketchup.com with the subject line "SketchUp Pro License" or similar. If the software is already installed and working