Magento 2 – Add Google reCaptcha to PayPal Payflow on Checkout Page

My currency setup was Indian rupee and it seems paypal doesn't support Indian currency. When I changed it to US Dollar. paypal option become visible.

I was actually moving to US Dollar, so this is no problem for me.

PS: The answer was given by user jspoe from magento room on IRC chat(freenode).

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.