Keyboard.h not found
If you have Arduino/Genuino Uno selected as your board in the Tools ► Board ► menu, then you get the error. If you select Arduino Leonardo it compiles correctly.

From Can't seem to be able use Keyboard.h library, this post states:


  Keyboard.h (and Mouse.h) only works on devices that use the ATmega32u4 processor, like the Leonardo or Micro.  See: https://www.arduino.cc/en/Reference/MouseKeyboard


This solution might help though


  I had the same problem using Flora
  
  This worked for me:
  
  
  Tools > Boards > Boards Manager
  Install TeeOnArdu (Then select that board from Boards Manager)
  Tools > USB Type > Serial, Keyboard, Mouse &lt; Joypad
  Upload and run code again (might need to restart IDE)