You installed and activated your 2021 license late November, just under a year ago. Doing that will have made your 2020 license be deactivated. Any existing 2020 installs would still work, but you wouldn’t be able to activate 2020 on a new machine.
Your 2021 license is still active, you can install 2021 and your 2021 license, and SketchUp will work. You do not need to pay for a subscription. Unless there is a reason for using 2020, you could reinstall 2021, check to see if the license is still there, and if it isn’t, add the license that the License Manager sent to you, the one that starts with WA- and has an authorization code that starts with ac7.
If you do also want to use 2020 on the computer you previously used it on, installing 2020 should still find the old license, and it ought to work too.