.error {
  border: 1px solid black;
  display: inline-block;
  margin-left: 20%;
  margin-right: 20%;
  position: fixed;
  top: 0;
  width: 100%;
}​

.error {
  border: 1px solid black;
  margin-left: 20%;
  margin-right: 20%;
  top: 0;
}​

<div class="fixed">
    <div class="error">Error!!!</div>
</div>

.error {
  border: 1px solid black;
  margin-left: 20%;
  margin-right: 20%;
}


.fixed {
    position: fixed;
    top: 0;
    left:0;
    right:0;
}