Getting `Uncaught TypeError: firebase.database is not a function`

Question

I'm trying to connect my contact form to Firebase, but keep getting this error saying "firebase.database is not a function", I see it's a common error but nothing I tried worked. I've troubleshoot it but don't know where to go around then ref

// Your web app's Firebase configuration
var firebaseConfig = {
    
  };
  // Initialize Firebase
  firebase.initializeApp(firebaseConfig);
  firebase.analytics();

// Reference messages collection

var messagesRef = firebase.database().ref('messages');

//listen to form submit
document.getElementById('contactForm').addEventListener('submit', submitForm);

//submit form
function submitForm(e){
    e.preventDefault();
    
    //Get values
    var fname = getInputVal('firstName');
    var lname = getInputVal('lastName');
    

    // save message
    saveMessage(fname, lname);
    
}

// Function to get Form values

function getInputVal(id){
    return document.getElementById(id).value;
}

// save messages to firebase


function saveMessage(fname, lname){
    var newMessageRef = messagesRef.push();
    newMessageRef.set({
        fname:fname,
        lname:lname
    });
}