, you are likely trying to reactivate a "Classic License" on an older machine. Since SketchUp moved to a subscription model years ago, managing these older perpetual licenses can be tricky. Finding Your Serial Number and Authorization Code
Troubleshooting Your SketchUp Pro 2015 License If you are looking for your SketchUp Pro 2015 serial number and authorization code
: If you still have the software running on an old computer, go to Help > License (Windows) or SketchUp > License (Mac) to view and copy your serial number. How to Authorize SketchUp Pro 2015 To activate your copy on a compatible machine: Open SketchUp 2015 and select Add Classic License on the Welcome screen. In the License modal, ensure the tab is selected. Manually type your Serial Number Authorization Code