Make a shortcut to the 2021 installer - which you can download from here
https://download.sketchup.com/SketchUpPro-2021-1-332-116.exe
Right click it and edit the target so it says -remove -silent
in my example I’ve put the installer straight into the C:
I’ve right clicked and chosen “create shortcut”
and then put the -remove -silent on the end
i.e.
C:\SketchUpPro-2021-1-332-116.exe -remove -silent
Alternatively, you can paste that into an admin elevated cmd prompt
Run the shortcut or the cmd prompt and it it will uninstall invisibily without complaining - this will save having to remove any other versions