Trivial attempt fails with "CoreUI's sidebar require Perfect Scrollbar"

I’ve tried a trivial example consisting of just the “Starter template” and pasted the body from the “Default layout” example into it. If I run it, I’m getting an error “CoreUI’s sidebar require Perfect Scrollbar”.

I’m not sure what I’m missing. The (somewhat reduced) code is:

<!doctype html>
<html lang="en">
        <!-- Required meta tags -->
        <meta charset="utf-8">
        <meta name="viewport" content="width=device-width, initial-scale=1">
        <!-- CoreUI CSS -->
        <link rel="stylesheet" href="" crossorigin="anonymous">
    <body class="c-app">
        <div class="c-sidebar">
            <!-- Sidebar content here -->
        <!-- Popper.js first, then CoreUI JS -->
        <script src=""></script>
        <script src=""></script>