sayHello(); error: Uncaught ReferenceError: sayHello is not defined

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

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>

sayHello();