@dave r said:

Please read the first three lines of the first post of this thread. It tells you exactly why you are getting this error. You need to set full permissions for the Plugins folder.

Many on work computers do not have the user permissions to modify the Plugins folder to "full permissions", and company policy will not allow the IT Administrator to grant this level of permissions.

On my work computer / network, just copy / pasting into the plugins folder requires that I type in a once per occurence User Account Control username and password (but this works for installing many plugins).
Same for any rbz plugin installation from within Sketchup.

Round Corner appears not to work unless you truly have the User Permissions modify the permississions of the plugin folder.

I don't and can't get this permissions level, and so Round Corner doesn't work for me.

Many other plugins install and work just fine without this permissions requirement.