I assume you are attempting to run the csc.exe executable from the command prompt - if so, that error means the path you are giving the command prompt is the wrong one. So if for example 'csc.exe' is stored on your C drive you would specify 'c:\csc.exe'.
What usually causes this is you have a space or something in one of the directories in your path and you are not enclosing the path in quotation marks
So instead of
you should be using
oooor if you are just typing in "csc" as stated in the msdn article here
then your computer is saying that it doesn't know where csc is and you have to either set it up to know where it is, or just specify the path of csc yourself.