As the name of getElementsByClassName suggest, it returns elementS, so you are getting HTMLCollection, not one element. Query the first one, like this:

function hideshowmenu() {
  var element = document.getElementsByClassName("box")[0];
  element.classList.toggle("bg-red");
}

or:

function hideshowmenu() {
  var elements = document.getElementsByClassName("box");
  elements[0].classList.toggle("bg-red");
}

What is returned by document.getElementsByClassName("box"); is a collection. You need to specify the index to access the classList property.

You are doing:

var element = document.getElementsByClassName("box");
element.classList.toggle("bg-red");

When you should be doing

var element = document.getElementsByClassName("box");
element[0].classList.toggle("bg-red");

You could also loop through the elements if you want to toggle multiple values.

Example code fiddle https://jsfiddle.net/agy5jtb0/1/

getElementsByClassName("box") return a collection of elements, but you considered it as a single element that is your mistake. I suggest something like this:

html:

<div class="mainmenu " onclick="hideshowmenu(this)">RED1</div>
<div id="submenu" class="submenu">
    <div class="mainmenu" onclick="hideshowmenu(this)">RED2</div>
    <div id="box" class="box"></div>
</div>

js:

function hideshowmenu(el) {
    el.classList.toggle("bg-red");
}