First, it would be good to indicate on which platform, Windows or Mac, you run Sketchup, either in your profile or in your message.

Second, you should indicate where you get these error messages: in a dialog box at startup, in the Ruby console.

Third, there should be line numbers in the message referring to the rb file. They are useful to troubleshoot.

Fourth, you should describe the circumstances of the problem: Is this after a new installation of Sketchup 8? Or a new installation of LibFredo6? or an upgrade of LibFredo6?

I can run LibFredo6, FredoScale and FredoTools with no problem on Sketchup 8 (Windows). Note that FredoCorner and Souvenir only support Sketchup 2016 and above.

So your errors may be due to a problem with the installation of those in Sketchup 8.

Fredo