Although you have activated on three machines, I only see information for two of them. Not sure why that is.
When you upgrade an old license, you can then only activate the latest version you have a license for. In your case that’s SketchUp 2021. If 2020 is already activated on a machine, it will continue to work, but you can’t activate 2020 on a new machine.
You did have 2020 activated on another machine. If you can find that one it will still be working.