there are probably other better ways to acomplish that but here is how to fix you script:

function cars() {
  document.querySelectorAll('.e38').forEach(function(e) {
    e.onclick = function(evt) {
      console.log('clicked', evt.target.innerHTML);
      document.getElementById("e38value").classList.toggle("e38hidden");
    }
  })
}

cars();

Note:

1. You need to call cars() to actually initialise the events.
2. The selector should be ".e38" which means all elements with the e38 class
3. You need to call forEach as the querySelector returns an array of elements (in that case with only one element)
4. The click event can be used to locate the target (where it was clicked).

Here is a fiddle: https://jsfiddle.net/b9728Lmk/1/

Does that solve the problem?

You can simply define an onchange event which will remove the visibility of the previously selected item and make the currently selected item visible.

function selectSection(value) {
    let context = document.getElementById("sections");
    for (let visible of context.querySelectorAll("p.visible")) {
        visible.classList.remove("visible");
    }
    if (context.children[value]) context.children[value].classList.add("visible");
}

#sections > p:not(.visible) {
    display: none;
}

<select id="section-selector" onchange="selectSection(this.value)">
    <option value="">Please Select</option>
    <option value="0">First</option>
    <option value="1">Second</option>
    <option value="2">Third</option>
    <option value="3">Fourth</option>
    <option value="4">Fifth</option>
</select>

<div id="sections">
    <p>section 1</p>
    <p>section 2</p>
    <p>section 3</p>
    <p>section 4</p>
    <p>section 5</p>
</div>

Here is an updated version jsFiddle

var  menuItemToClick = document.querySelectorAll('.e38')


function cars(){
// I use for here because the menuItemToClick is an array that containt all element having class name e38
  for (var i = 0; i < menuItemToClick.length; i++) {
      menuItemToClick[i].addEventListener('click', function(event) {
          console.log('clicked');
          ParagraphToHide = document.querySelectorAll('#e38value');
          //I use [0] because the ParagraphToHide is an array, as we use an id we are pretty sure that we will select the first element 
          ParagraphToHide[0].classList.toggle("e38hidden");
      });
  }
} 

cars();