Sorry to hear that you are experiencing issues with your OctaneRender for Unity installation.
Just to confirm - have you closed and reopened Unity, before re-attempting the installation process? In the event that hiccups like this occur, a closing and re-opening Unity can resolve it more often than not.
If this does not resolve the issue, we can expand the troubleshooting process from there. I have attempting the same installation process on my own computer, with the same version of Unity already installed, and have so far been unable to replicate the issue.
I look forward to getting you up and running again as soon as possible.