Use the default PMPro checkout page template instead of a custom theme tempalate.

my_pmpro_checkout_custom_template_path_use_default.php

<?php

/**
 * Use the default PMPro checkout page template instead of a custom theme tempalate.
 *
 * 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_checkout_custom_template_path_use_default( $default_templates, $page_name, $type, $where, $ext ) {
	// Add a custom template path for the checkout page.
	$default_templates[] = PMPRO_DIR . "/{$type}/{$page_name}.{$ext}";
	return $default_templates;
}
add_filter( 'pmpro_pages_custom_template_path', 'my_pmpro_checkout_custom_template_path_use_default', 10, 5 );