It means that there is somewhere a class called Key and somewhere else is also a class called Key.
I do not get your error (yet) and after fixing your warnings/errors it compiles. First I suggest that you go to file -> preferences and set compiler warnings to ALL so you can see all the things that the compiler does not trust.
I suggest that you start by looking at and understanding (!!) the custom keypad example; the understanding is important. Once you have corrected the lines indicated by @anon73444976, you can start studying the examples so you can fix the errors / warnings that relate to that.
Notes:
Text between single quotes is a character constant and can therefore only contain one character. Both e.g. 'pinMode(11,INPUT_PULLUP)' and '/0' are multiple characters; for the latter, there is a difference between '\0' and '/0'.
Once you have fixed that, the compiler will complain about a missing variable called display. Again, study the examples (for the Adafruit_SSD1308 library this time) to figure that out.
Your topic has been moved to a more suitable location on the forum. Installation and Troubleshooting is not for problems with (nor for advise on) your project 
See About the Installation & Troubleshooting category.