i have no idea about this error, because i just checked all of other topics and didn't get result.
let demo = document.getElementById("#demo");
demo.innerText("yo");
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Pract</title>
</head>
<body>
<script src="javascript.js"></script>
<p id="demo"></p>
</body>
</html>