A scroll bar is only visible if a content is bigger than its scroll-able container.

E.g. here, the content's height is 800px but its container is only 400px:
(Please provide more context if this doesn't answer your question)

.container{
  overflow-y: scroll;
  
  width: 200px;
  height: 400px; /* Half of its content */
}

<div class="container">
  <img src="https://placehold.co/200x800">
</div>