Redirect users without membership access from LifterLMS single course pages to membership checkout.

my_pmpro_lifterlms_single_course_redirect_non_members.php

<?php
/**
 * Redirect users without membership access from LifterLMS single course pages to membership checkout.
 *
 * 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_lifterlms_single_course_redirect_non_members() {
	// Bail if PMPro isn't active.
	if ( ! function_exists( 'pmpro_has_membership_access' ) ) {
		return;
	}

	// Bail if we're not on a Lifter LMS single course page.
	if ( ! is_singular( 'course' ) ) {
		return;
	}

	// Check the user's membership access and get required memberships.
	$access = pmpro_has_membership_access( null, null, true );

	// Bail if we're unable to get access information.
	if ( ! is_array( $access ) ) {
		return;
	}

	// Get access status.
	$has_access = $access[0];

	// Bail if the user has access.
	if ( $has_access ) {
		return;
	}

	// Get required membership level IDs.
	$required_membership_levels_ids = $access[1];

	// Redirect users without access to the checkout page for the first required membership level.
	$checkout_url = pmpro_url( 'checkout', '?pmpro_level=' . $required_membership_levels_ids[0] );
	wp_redirect( $checkout_url );
	exit;
}
add_action( 'template_redirect', 'my_pmpro_lifterlms_single_course_redirect_non_members' );