Actually, the published version of the Legacy RoundCorner seems to be for prior SU2016, which creates this issue (with LibFredo6 being prior SU2017).
A quick fix is to
go to the Plugin folder Edit the file Fredo6_RoundCorner.rb on the top there is a lineif Sketchup.version.to_i < 16. Replace the 16 by 17 and save the file. Restart Sketchup
I'll see if I can re-release this legecy version of RoundCorner.