With a subscription license, as your profile indicates you have, there is no serial number no authorization code. The e-mail address and password you used when you purchased the subscription serves that purpose. Try signing out and signing in again. That generally takes care of it.
After following these directions: Authorizing SketchUp with a Laptop License (Educational Institutions) SketchUp Help to authorize sketchup using a license, launching sketchup and clicking add license does not pre-populate the serial number and auth code.
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