JSFiddle - React, Tailwind, and code Playground

HTML

<script src="//code.jquery.com/jquery-1.11.2.min.js"></script>
<label>
    <input type="checkbox" name="agreement" class="agreement"> I read all the information in the red box above and followed the instructions!
</label>
<br/>
<input disabled="disabled" type="submit" name="submit" class="send">

JavaScript

$(".agreement").on('click', function() {
   if($(this).is(':checked')){
		$(".send").prop('disabled',false);
	} else {
		$(".send").prop('disabled',true);
	}
})