please help slove this i can't execute my appex class code
Whenever i try to execute my program i get this error .

public class Demo1 {
    
    public void printOutput(String stringToDisplay){
    System.debug('Display text: ' + stringToDisplay);
}
} 

Demo1 d1 = new Demo1();
d1.printOutput('Hello World');

this what i am trying execute.