In the SketchUp 2019 client, the use of the Serial Number and Auth code is a licensing option for those who are not using a subscription. All you should need to do is sign in.
Be sure that your subscription seat is assigned to you. You can do so by going to your Account Management Portal and enter into the MEMBERS area to make sure you are assigned your seat. Make sure that you have the appropriate subscription type selected. Here is a graphic to help explain:
More information can be found here in the SketchUp Help Center
Hope this helps
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