The error message says what the problem is: Cannot find anything named "linein".
Indeed, there is nothing else named "linein".
After that, you'll find there is nothing named "getLinein" and nothing named FFT.HAMMinG
Is C# case-insensitive? I did not know that...
After that, you'll find that you need to remove references to isPlaying() and in.position(), which aren't relevant if you aren't playing a sample.
More bugs after that.
For what it's worth, I have some working FFT code (that works with both a sample, or with linein) you can download here: