Add the HTML5 required attribute to all checkout fields that has the pmpro_required class for front-end validation that checks that a required field is not empty before processing the form submit.

my-pmpro-add-required-attribute-to-input.php

<?php
/**
 * This recipe adds the HTML5 attribute "required" to all required
 * fields on the checkout page that has the class "pmpro_required"
 * to check that a required field is not empty before submitting.
 *
 * @link https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/required
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_add_required_attribute_to_input() {
	if ( pmpro_is_checkout() ) {
		?>
		<script>
		jQuery(document).ready(function ($) {
			jQuery('.pmpro_required').attr('required', 'required');
		});
		</script>
		<?php
	}
}
add_action( 'wp_footer', 'my_pmpro_add_required_attribute_to_input' );