JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//cdnjs.cloudflare.com/ajax/libs/jquery-validate/1.11.1/jquery.validate.js"></script>
<form  id="testForm" method="get" action="">
  <fieldset>
    <p>
      <label for="MaxAVMPG">MaxAVMPG</label>
      <input id="MaxAVMPG" name="MaxAVMPG" type="text" required/>
      <div id="MaxAVMPGchk"></div>
    </p>
    <p>
      <label for="email">email</label>
      <input id="email" name="email" type="email" required/>
    </p>
    <p>
      <input class="submit" type="submit" value="Submit"/>
    </p>
  </fieldset>
</form>

JavaScript

function isNumber(str) {
   var pattern = /^\d+$/;
   return pattern.test(str);  // returns a boolean
}

$(function() {
    $('#MaxAVMPG').keyup(function() {
        if (!isNumber($('#MaxAVMPG').val())) {
            $('#MaxAVMPGchk').html('Please enter valid number');
        } else {
            $('#MaxAVMPGchk').html('');
        }
    });

});