why is keyboard.h library not working on arduino uno

this is the code:
#include <Keyboard.h>

int o=10;

void setup() {
// put your setup code here, to run once:
pinMode(o,INPUT_PULLUP);
Keyboard.begin();
}

void loop() {
// put your main code here, to run repeatedly:
if(digitalRead(o)==HIGH){
Keyboard.write(111);
delay(200);}

else{
Keyboard.release(111);
}
Keyboard.end();
}

and this is the error:

‘Keyboard’ not found. Does your sketch include the line ‘#include <Keyboard.h>’?

why is it not working?
i have written #include <Keyboard.h> but it is not ‘‘noticing’’ it.
can please someone help me??

https://www.arduino.cc/en/Reference/MouseKeyboard

Always check the reference. Pay particular attention to that first sentence. HINT: the UNO uses a 328P.

Also, it is not recommended you use single character variables.

...but it is recommended that you use code tags when posting code. But you don't need to post code when you've pre-Googled the problem.

It would be nice if one of the USB include files had a #error for compiling on a non-USB platform. :(