Why is .textContent giving me an Uncaught Typeerror: Cannot set properties of null (setting ‘textContent’)?? [duplicate]

While making a BlackJack game, In line 25 in JS I am trying to tell JavaScript to redeclare the .textContent of the variable messageEl to the variable message whenever the user clicks on the button but it’s giving the error Uncaught Typeerror: Cannot set properties of null (setting ‘textContent’)

HTML:

<h1>Blackjack</h1>
<p>Want to play a round?</p>
<p>Cards: </p>
<p>Sum: </p>
<button onclick="startGame">START GAME</button>

Javascript:

 let firstCard = 2
 let secondCard = 1
let sum = firstCard + secondCard

let isAlive = true
let hasBlackJack = false

let message = ""

let messageEl = document.getElementById("messageEl")


function startGame() {
if (sum <= 20) {
   message = "Do you want a draw?"
} else if (sum === 21) {
hasBlackJack = true
    message = "Whoa! You've won a BlackJack" 
} else {
isAlive = false
message = "You lost"
}
messageEl.textContent = message // this is the line that's making the problem and I have no clue what makes it wrong!
}