Set tweet to the value of the textarea element.

<html>
<head><style>body {min-height: 300px;} input { font-size: 16px; } textarea { font-size: 16px; } </style></head>
<body>
<img id="profilePic" src="https://mimo.app/r/profile.png" height="50">
<textarea id="tweetText" rows="3" cols="18" placeholder="What's happening?"></textarea>
<br>
<button onclick="tweetPost()">Tweet</button>

  <script>function tweetPost() {
  var tweet = document.getElementById("tweetText").value;
  console.log(tweet);
}</script>
</body>
</html>]]>

Use .value to get the value of a textarea element.