Bootstrap toast does not show up

You need to put the valid option. i:e show, hide or a callback function. See - https://getbootstrap.com/docs/4.2/components/toasts/.

$('.toast').toast('show');

<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" rel="stylesheet"/>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>
<div class="toast" role="alert" aria-live="assertive" aria-atomic="true">
  <div class="toast-header">
    <img height="200px" width="200px" src="https://upload.wikimedia.org/wikipedia/commons/f/f9/Phoenicopterus_ruber_in_S%C3%A3o_Paulo_Zoo.jpg" class="rounded mr-2" alt="...">
    <strong class="mr-auto">Bootstrap</strong>
    <small class="text-muted">11 mins ago</small>
    <button type="button" class="ml-2 mb-1 close" data-dismiss="toast" aria-label="Close">
                <span aria-hidden="true">&times;</span>
                </button>
  </div>
  <div class="toast-body">
    Hello, world! This is a toast message.
  </div>
</div>

If you do not want to close the toast automatically, add data-autohide="false".

$('.toast').toast('show')

<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" rel="stylesheet"/>

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>
<div class="toast" data-autohide="false">
  <div class="toast-header">
    <svg class=" rounded mr-2" width="20" height="20" xmlns="http://www.w3.org/2000/svg" preserveAspectRatio="xMidYMid slice" focusable="false" role="img">
                    <rect fill="#007aff" width="100%" height="100%" /></svg>
    <strong class="mr-auto">Bootstrap</strong>
    <small class="text-muted">11 mins ago</small>
    <button type="button" class="ml-2 mb-1 close" data-dismiss="toast" aria-label="Close">
                    <span aria-hidden="true">&times;</span>
                  </button>
  </div>
  <div class="toast-body">
    Hello, world! This is a toast message.
  </div>
</div>

<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" rel="stylesheet"/>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/js/bootstrap.min.js"></script>


<div class="toast" data-autohide="false">
    <div class="toast-header">
        <svg class=" rounded mr-2" width="20" height="20" xmlns="http://www.w3.org/2000/svg" preserveAspectRatio="xMidYMid slice" focusable="false" role="img">
        <rect fill="#007aff" width="100%" height="100%" /></svg>
        <strong class="mr-auto">Bootstrap</strong>
        <small class="text-muted">11 mins ago</small>
        <button type="button" class="ml-2 mb-1 close" data-dismiss="toast" aria-label="Close">
            <span aria-hidden="true">&times;</span>
        </button>
    </div>
    <div class="toast-body">
    Hello, world! This is a toast message.
    </div>
</div>

You need to start " $(".toast").toast('show'); " when document is ready.

<script>
    $(document).ready(function() {
        $(".toast").toast('show');
    });
</script>

Firstly, you need to make the toast fixed to top or bottom, depending on where you want it.

<div role="alert" aria-live="assertive" aria-atomic="true" class="toast fixed-bottom m-5 ms-auto"
    data-bs-autohide="false">
    <div class="toast-header">
        <img src="..." class="rounded me-2" alt="...">
        <strong class="me-auto">Bootstrap</strong>
        <small>11 mins ago</small>
        <button type="button" class="btn-close" data-bs-dismiss="toast" aria-label="Close"></button>
    </div>
    <div class="toast-body">
        Hello, world! This is a toast message.
    </div>
</div>
<script>
    window.onload = (event) => {
        let myAlert = document.querySelector('.toast');
        let bsAlert = new bootstrap.Toast(myAlert);
        
        setTimeout(function () {
            bsAlert.show();
        }, 5000);
    };
</script>