It looks like you are trying to retrieve the 'toast' value from the session, but it is not being set properly in the first place.
Make sure that the 'toast' value is being set correctly in the session when you redirect back. You can check this by adding a dd(session('toast')) after the redirect in your controller.
If the value is not being set, try using the with() method like this:
return redirect()->back()->with(['toast' => 'Test Toast']);
If the value is being set correctly, make sure that you are retrieving it correctly in your view. You can try using the session() helper function directly in your view like this:
@if(session('toast'))
<div class="toast">{{ session('toast') }}</div>
@endif
Also, make sure that you are passing the 'toast' value to your view correctly. You can try using the with() method like this:
return Inertia::render('YourView')->with('toast', session('toast'));