Any idea what this error means

This is my first ever project im making a basic fps game and I have a character object and the ground, however I’m getting this error
Version free ,4.3 mac
Internal compiler error. See the console log for more information. output was:
Unhandled Exception: System.IO.FileNotFoundException: Could not load file or assembly ‘System.Runtime, Version=, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a’ or one of its dependencies.

I’m not using mac, but it is stated clearly that it could no load System runtime. Try to reinstall unity, also install 4.6 instead of 4.3 because it fixes many bugs in unity