Magento – How to add google reCaptcha on the checkout page Magento 2

To do so, you need some custom code, please follow below steps which work for me:

Note: I assume you used jQuery library on your frontend theme

Step-1: On document ready add some custom class to your reCAPTCHA input field:

<script type="text/javascript">
jQuery(document).ready(function(){
    jQuery('#recaptcha_response_field').addClass('required-captcha-entry');
})
</script>

Step-2: Initialize Magento VarienForm javascript class object and add new validation rule for reCAPTCHA field:

<script type="text/javascript">
var yourFormValidationObj = new VarienForm('your_form_id');

Validation.add('required-captcha-entry', ' ', function(v) {
    return !Validation.get('IsEmpty').test(v);
})
</script>

Step-3: On form submit, validate form for required values

<script type="text/javascript">
if(yourFormValidationObj.validator.validate()){
    // Form validate and ready to submit
    jQuery('YOUR_FORM_IDENTIFIER').submit();
} 
</script>

That's, your reCAPTCHA field is become required field and validated before submit the form.

You should try this code: I have been using this on my site.

<script>
window.onload = function() {
  var recaptcha = document.forms["contactForm"]["g-recaptcha-response"];
  recaptcha.required = true;
  recaptcha.oninvalid = function(e) {

    alert("Please complete the captcha");
   }
}
</script>