Alright, I want you to do two things for me. First, look at the the screenshots I've just posted here. I'm running a Mac, but don't worry if you're on Windows - the details will be the same.
First, make sure that your folder structure look just like mine does. In my Arduino folder, with all my sketches, I have the 'libraries' folder. In side of that, I have the 'Adafruit_NFCShield_I2C' folder, and that folder contains both the Adafruit_NFCShield_I2C.cpp and Adafruit_NFCShield_I2C.h files.
Notice how the name of the folder, the name of the files, and the name of the thing that's giving you the error are all spelled exactly the same? They are all spelled 'Adafruit_NFCShield_I2C'.
Make sure you've got the same folder structure.
Second, I want you to run your Arduino application, and open the 'Preferences' window. Again, I've got a Mac, but the Preferences will look pretty much the same in Windows.
At the top of the window, you'll see a box labelled "Sketchbook Location". That box must have a path to the same Arduino folder which contains your libraries folder with the NFC library, just like it does in my picture.
So, go through and make sure your Adafruit_NFCShield_I2C folder is in the right place, has the right name, and contains the right files.
Close down your Arduino application, and then restart it.
Try your NFC sketch.
If it still doesn't work, then I want you to make a screenshot of your directories, and a screenshot of your Preferences window, just like mine, and post them here.
I also want you to copy and paste any error messages you get.
It really is not enough for you to just tell us that you "got an error". We need to see the exact text of the errors, even if they just look like gibberish.