Conditional Statement wont run !

I created an input section where the username is entered and the conditional statement checks if user name === 'USERNAME2000' then it appends some text to the DOM. But the console keeps telling me that i cant add .toUpperCase() to my condition? i dont know why This is my code

// DOM ELEMENTS let container = document.querySelector('.container'); let container2 = document.querySelector('.container-fluid'); container2.style.display = 'none'; let button = document.querySelector('.btn'); //BUTTON EVENT LISTENERS button.addEventListener('click', () => { let userName = document.getElementById('userName').value;

if (userName.toUpperCase() === 'USERNAME2000') {
    let h2 = document.createElement('h2');
    h2.innerHTML = 'another one';
    container.append(h2);
} else {
    alert('Thats not the username');
}

})