If you get an error, ensure you are trying to activate version 2020, not a newer or older version. Remove License: If you are moving to a new computer, you must first go to Help > License and choose Remove License on the old machine to free up the activation.
Ensure you have the SketchUp Pro 2020 installer. License keys are version-specific; a 2020 key will not work on 2019 or 2021 versions.
Look for an email from sketchupmail@sketchup.com with the subject line containing "SketchUp Pro License Information".
(also called a license key) provided via email at the time of purchase. Key Features and Limitations One-Time Purchase
If the email is gone, use the SketchUp License Manager . You’ll need the email address used for the original purchase to look them up.