let checkbox;

function setup() {
  noCanvas();
  // There's no need to use createCheckbox() since you're create the <input /> tag yourself
  // Also I got rid of the wrapping div since it wasn't strictly necessary.
  let label = createElement(
    'label',
    `<input id="toggle" type="checkbox" />
     <span class="slider round"></span>`
  );
  
  // Because I'm using createElement() to create the label I have to add the class attribute:
  label.addClass('switch');

  checkbox = select('#toggle');
  // Register the event handler after using select() to get the p5.Element for the checkbox.
  checkbox.changed(myCheckedEvent);
}

function myCheckedEvent() {
  if (this.checked()) {
    console.log("Checking!");
  } else {
    console.log("Unchecking!");
  }
}

html, body {
  margin: 0;
  padding: 0;
}

/* The switch - the box around the slider */
.switch {
  position: relative;
  display: inline-block;
  width: 60px;
  height: 34px;
}

/* Hide default HTML checkbox */
/* COMMENT: there was a typo here, uses to be .siwtch */
.switch input {
  opacity: 0;
  width: 0;
  height: 0;
}

/* The slider */
.slider {
  position: absolute;
  cursor: pointer;
  top: 0;
  left: 0;
  right: 0;
  bottom: 0;
  background-color: #ccc;
  -webkit-transition: .4s;
  transition: .4s;
}

.slider:before {
  position: absolute;
  content: "";
  height: 26px;
  width: 26px;
  left: 4px;
  bottom: 4px;
  background-color: white;
  -webkit-transition: .4s;
  transition: .4s;
}

#toggle:checked + .slider {
  background-color: #2196F3;
}

#toggle:focus + .slider {
  box-shadow: 0 0 1px #2196F3;
}

#toggle:checked + .slider:before {
  -webkit-transform: translateX(26px);
  -ms-transform: translateX(26px);
  transform: translateX(26px);
}

/* Rounded sliders */
.slider.round {
  border-radius: 34px;
}

.slider.round:before {
  border-radius: 50%;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/p5.js/1.4.0/p5.js"></script>