How to add arrow to div?

You can use CSS Pseudo classes to do this, therefore you won't need an "arrow" element:

div.error-label {
  padding: 10px;
  color: #fff;
  background-color: #DA362A;
  -webkit-border-radius: 5px;
  -moz-border-radius: 5px;
  border-radius: 5px;
  width: 190px;
  margin: 30px;
}

div.error-label:before {
  content: ' ';
  height: 0;
  position: absolute;
  width: 0;
  left: 18px;
  border: 10px solid transparent;
  border-right-color: #DA362A;
}
<div class="error-label">This field is required.</div>


Here is the solution

div.error-label{
    padding: 7px 10px 0 4px;
    color: #fff;
    background-color: #DA362A;
    -webkit-border-radius: 5px; 
    -moz-border-radius: 5px; 
    border-radius: 5px;
    width: 150px;
    margin-left: 5px;
}

div.error-label-arrow{
    border-top: 10px solid transparent;
    border-bottom: 10px solid transparent; 
    border-right:10px solid #DA362A;
    float: left;
    height: 1px;
    margin-top: 3px;
}
<div class="error-label-arrow"></div>
<div class="error-label">This field is required.</div>


You can achieve this by using Pseudo elements check below answer

.error{
   position: relative;
   background: red;
   padding: 10px;
   color: #fff;
   margin:0 0 0 12px;
   width:auto;
   display:inline-block;
 
 }

.error::before {
	content: "";
	width: 0;
	height: 0;
	border-style: solid;
	border-width: 9px 11px 9px 0;
	border-color: transparent red transparent transparent;
	position: absolute;
	left: -11px;
	top: 0;
	bottom: 0;
	margin: auto;
}

.arrow_type2 {
	position: relative;
	background: #28d57f;
	border: 3px solid #0cf5a7;
  padding:50px 20px;
  margin-left:20px;
  margin-top:20px;
  width:auto;
  display:inline-block;
}
.arrow_type2:after, .arrow_type2:before {
	right: 100%;
	top: 50%;
	border: solid transparent;
	content: " ";
	height: 0;
	width: 0;
	position: absolute;
	pointer-events: none;
}

.arrow_type2:after {
	border-color: rgba(40, 213, 127, 0);
	border-right-color: #28d57f;
	border-width: 10px;
	margin-top: -10px;
}
.arrow_type2:before {
	border-color: rgba(12, 245, 167, 0);
	border-right-color: #0cf5a7;
	border-width: 14px;
	margin-top: -14px;
}
<div class="error">This is field required</div>


<div class="arrow_type2">Arrow type two</div>