JSFiddle - React, Tailwind, and code Playground

HTML

<label for="position"><b>Position:</b></label><br/><br/>
<input type="radio" name="position" id="r1" value="Dean" onclick="Checkradiobutton()"/>Dean
<input type="radio" name="position" id="r2" value="Instructor" onclick="Checkradiobutton()"/>Instructor
<input type="radio" name="position" id="r3" value="Student" onclick="Checkradiobutton()"/>Student<br/><br/>
<input type="radio" name="position" id ="r4" value="<?php echo $_POST['otherPosition']?>" onclick="Checkradiobutton()"/>
Others: 
<input type="text"  value = "<?php if($error==TRUE){echo $_POST['otherPosition'];}?>" id="otherPosition" placeholder="Specify" name="position" />

CSS

.field-name
           {
          color: #444;
          font-family: verdana;
         font-size: 0.85em;
         line-height: 2em;
            }

JavaScript

function Checkradiobutton()
 {

  if(document.getElementById('r1').checked || document.getElementById('r2').checked || document.getElementById('r3').checked)
 {

        document.getElementById('otherPosition').disabled=true; 
   }else{
                    document.getElementById('otherPosition').disabled = false;
                }
 }