Problems with keyboard library

My programme has to write in the keyboard a letter when you press the botton. But when I compile it I get error. Why do I get this error if I have this library yet?

#include <Keyboard.h>


const int boton1= 6;


int buttonState = 0;





void setup() {
  pinMode(boton1, INPUT);


Keyboard.begin();
}



void loop() {
  if (digitalRead(boton1) == HIGH) {
    Keyboard.write('u');
  }

}

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You’re compiling for an Uno, which isn’t compatible with the Keyboard library:
https://www.arduino.cc/en/Reference/MouseKeyboard
If you actually have a different board such as Leonardo or Pro Micro that is compatible with Keyboard then you need to select that board from the Tools > Board menu, otherwise you’ll need to buy one of those boards before you can use the library.