Redirect logged out users from a specific post to the site's login page.

redirect_logged_out_users_from_post.php

<?php

// Copy below this line.

/**
 * Redirect logged out users from a specific post to the site's login page.
 * 
 * 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_redirect_logged_out_users_from_post() {
	if ( is_single( 20206 ) && ! is_user_logged_in() ) {
		wp_redirect( wp_login_url() );
	}
}
add_action( 'template_redirect', 'my_redirect_logged_out_users_from_post' );