JSFiddle - React, Tailwind, and code Playground

HTML

<input type = "text"  size="33"   class="uploadtxt1 url" name="txturl_<?php echo $testing_type_id ; ?>" id = "txturl_<?php echo $testing_type_id; ?>_<?php echo $i ; ?>" value="" />
             <input type = "text"  class="uploadtxt2" size="33" name="txtinstruction_<?php echo $testing_type_id ; ?>" id = "txtinstruction_<?php echo $testing_type_id; ?>_<?php echo $i ; ?>" value=""  /> <br/>

<input type = "text"  size="33"   class="uploadtxt1 url" name="txturl_<?php echo $testing_type_id ; ?>" id = "txturl_<?php echo $testing_type_id; ?>_<?php echo $i ; ?>" value="" />
             <input type = "text"  class="uploadtxt2" size="33" name="txtinstruction_<?php echo $testing_type_id ; ?>" id = "txtinstruction_<?php echo $testing_type_id; ?>_<?php echo $i ; ?>" value=""  /> <br/>
<input type = "text"  size="33"   class="uploadtxt1 url" name="txturl_<?php echo $testing_type_id ; ?>" id = "txturl_<?php echo $testing_type_id; ?>_<?php echo $i ; ?>" value="" />
             <input type = "text"  class="uploadtxt2" size="33" name="txtinstruction_<?php echo $testing_type_id ; ?>" id = "txtinstruction_<?php echo $testing_type_id; ?>_<?php echo $i ; ?>" value=""  /> <br/>

             <input type="submit" name="checkout" id="checkout"   class="graybtn1 ptr  button_float_left_checkout" disabled="disabled" value="Proceed To Checkout" />

JavaScript

$(document).ready(function(){
    
    $("input[class^='uploadtxt']").keyup(function(e){
        var alltxt=$("input[class^='uploadtxt']").length;
        var empty=true;
        $("input[class^='uploadtxt']").each(function(i){
            if($(this).val()=='')
            {
                empty=true;
                $('#checkout').prop('disabled', true);
                return false;
            }
            else
            {
                empty=false;
            }
        });
        if(!empty)  $('#checkout').prop('disabled', false);                    
        
     });
    
});