A QuickLook generator is included with SketchUp 2020 and ought to be working (it is working for me under Catalina). To verify that it is registered on your system, you can open a Terminal window and enter
qlmanage -m | grep SketchUp
You should see a list of various com.sketchup.xxx entries such as com.sketchup.skp with an arrow pointing to the path within the SketchUp 2020 Application where the qlgenerator lives. If that doesn’t work, try
qlmanage -r
to reset the quicklook system manager and then try the first command again.