Bootstrap Dual Listbox scroll bar is missing

Question

I have created a web application using ASP.NET MVC 5 and Bootstrap 5 and I am using bootstrap dual listbox for the selection of countries. I have no problem on the functions of dual listbox but the scroll bar is missing so I can't navigate to the bottom items. The only way is using filter.

Can anyone please tell me why this is behaving like that.

Here is the image of my dual list box.

https://i.stack.imgur.com/FltWz.png

@Html.ListBoxFor(m =\> m.Region_code, new SelectList(ViewBag.country, "Value", "Text"), new { @class = "countryListBox", @multiple = "multiple", @id = "dualListbox", @Name = "dualListbox", @size = "10" })

$(document).ready(function () {
    $('.countryListBox').bootstrapDualListbox();
});