Arduino Error

I know this is off-topic but I really think you guys can help me.

I'm making an AI Guessing Game using an Arduino UNO, a breadboard, 5 buttons, and 5 LEDs.

However, there is an error I don't know how to fix:

The button source code was taken from here:
https://www.arduino.cc/en/Tutorial/BuiltInExamples/Button