Unfortunately since SketchUp 2021 is out, you will no longer be able to get a license to SU 2020. Keep in mind it is also subscription now, so you're going to keep on paying for the latest version of Sketchup every year. Stop paying and you lose access to your SketchUp work.
Your files from SU2015 will open and work just fine in the latest version of Sketchup. You will have to update your plugins / extensions to ones that are compatible with the latest version of SU.