This was the final version of SketchUp to support a "one-time buy" model. It uses a Serial Number and an Authorization Code provided via email at the time of purchase.

Most "free" software activators are wrappers for malicious code that can steal personal data or lock your files.

on the welcome screen and follow the browser prompts to authenticate your account. Key Licensing Features in 2021 Understanding Your Classic License | SketchUp Help

You can manage active seats and view your plan details by logging into the SketchUp License Manager Key Usage Rules Device Limit: A single-user license allows you to install SketchUp on two computers

Key: SKETCHUP-2021-ETHICAL-USE-ONLY. Use it. Finish your project. And remember: the cheapest way to pay for software is with money. Everything else costs more.