Navbar toggle cannot be displayed

I am using bootstrap to write this simple navbar hamburger toggle.
However on the page I can see only one bar on the left…
Which line of code went wrong?

Nevermind I found the mistake…

The video tutorial I followed was using bootstrap 3, and I am using bootstrap 4.
So I need to use <span class="navbar-toggler-icon"></span> in order to display the hamburger button.