Answers for "uncaught referenceerror function is not defined"

Uncaught ReferenceError: function is not defined at HTMLUnknownElement.onclick

//Your program can't find the function so do this.

//You can assign the button onclick as an id.
//then attach an eventListener on run time like this:

<button id="btn">Click me!</button>

var btn = document.getElementById("btn");
btn.addEventListener("click", function() {
	//Do something here
}, false);

Uncaught ReferenceError: $ is not defined

<script src="https://code.jquery.com/jquery-1.9.1.js"></script>
<script src="path-para-seu-script"></script>

Uncaught ReferenceError: $ is not defined

You should put the references to the jquery scripts first.
<script src="jquery-3.5.1.js"></script>
<script type="text/javascript" src="script.js"></script>