You have to add recaptcha external js so use following function in your current themes functions.php file

Set Recaptcha element in your form.

//Custom Form

Put below js in your current themes custom.js file.

//Captcha validation script
jQuery(document).ready(function($) {
$(document).on(‘submit’,’.wc-captcha-form’,function(e){
if(grecaptcha.getResponse() == “”) {
e.preventDefault();
$(‘.captcha-error’).text(“Please verify captcha”);
} else {
$(‘.captcha-error’).text(“”);
}
});
});