WordPress mu plugins to check if "Advanced Custom Fields Pro" is active.

acf-missing.php

<?php

add_action('plugins_loaded', function() {
    if ( !function_exists('is_plugin_active') ) {
        /** WordPress Plugin Administration API */
        require_once(ABSPATH . 'wp-admin/includes/plugin.php');
    }
    
    if ( is_plugin_active( 'advanced-custom-fields-pro/acf.php' ) ) {
        return;
    }

    $tp_acf_notice_msg = __( 'This website needs "Advanced Custom Fields Pro" to run. Please download and activate it', 'tp-notice-acf' );
    
    /*
    *	Admin notice
    */
    add_action( 'admin_notices', function () use ($tp_acf_notice_msg) {
        echo sprintf('<div class="notice notice-error notice-large"><div class="notice-title">%s</div></div>', $tp_acf_notice_msg);
    } );

    /*
    *	Frontend notice
    */
    add_action( 'template_redirect', function () use ($tp_acf_notice_msg) {		
        wp_die( $tp_acf_notice_msg );
    }, 0 );
});