Why does mobile navigation look like this?

Question

css

I'm building a website on OptimizedHTML4
I started making a menu, and I saw that it is not hidden on mobile devices.
Here is the standard code for bootstrap navigation.
Here is what is included in the project:

@import "../libs/bootstrap/scss/bootstrap-reboot.scss"
@import "../libs/bootstrap/scss/bootstrap-grid.scss"
@import "../libs/bootstrap/scss/_navbar.scss"
@import "../libs/bootstrap/scss/_nav.scss"
@import "../libs/bootstrap/scss/utilities/_sizing.scss"

<div class="container">
  <nav class="navbar navbar-expand-lg navbar-light bg-light">
  <a class="navbar-brand" href="#">Navbar</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>
  <div class="collapse navbar-collapse" id="navbarNav">
    <ul class="navbar-nav">
      <li class="nav-item active">
        <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Features</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="#">Pricing</a>
      </li>
      <li class="nav-item">
        <a class="nav-link disabled" href="#">Disabled</a>
      </li>
    </ul>
  </div>
</nav>
</div>