getElementById in chrome extension

I have written a code for extension but it doesn’t work: I can’t getElementById in document nor in extension popup. I would like when I click popup text extension starts the game.
HTML CODE;

    <!DOCTYPE html>
<html>
<head> 

<style> p {color: purple;} </style>
</head>
<body>
<p id="pocinje"> Ekstenzija je pokrenuta i ceka se vrijeme za pocetak. </p>
<script src="background.js"> </script>
</body>
</html>

JAVASCRIPT;

document.getElementById("pocinje").onclick= function() {open};

function open() {
document.getElementById("startNewGame").click();

console.log("alorka");

}

How can I fix this?

getElementById in chrome extension