Unable to make a really basic sketch work
#define button_pin 2  //defining button pin as pin 2
bool button_state = 0; //button state is either 0/1 so using bool
void Button();
void setup()
{
    Serial.begin(9600);
    pinMode(button_pin, INPUT_PULLUP); //Setting input as input pullup
}

void loop()
{
    Button();
}

void Button()
{
    
    button_state = digitalRead(button_pin); //Read button state
    if (button_state == LOW)
    {
        Serial.println(button_state); //print button state when it is low
    }
}

In your code, you have used = operator in place of == in the if loop. Just if you change it, it will start working also you can use my above code it is same as yours just I have made some minor changes such as using bool in place of int and use of #define in place of pin declaration where you used int