WHMCS UUID Generator for clients and admins. Tested working on WHCMS 7.4.1

generateWhmcsUuids.php

<?php
/**
 * Generate uuids for clients and admins that don't have uuids set.
 * 
 * The WHMCS 7.4.1 update process automatically inserts uuids, but if your 
 * installation bypasses WHMCS update routines then this will generate uuids for 
 * the client and admin users that don't have one yet.
 * 
 * Warning! Please back up at least your tblclient and tbladmin tables before 
 * running this!
 */

use Ramsey\Uuid\Uuid;
use WHMCS\User\Admin;
use WHMCS\User\Client;

require_once dirname(__FILE__)."/../init.php";

// Look for client and admin users with empty uuids.
$clients = Client::where('uuid', '')->get();
$admins = Admin::where('uuid', '')->get();

var_dump('Found ' . $clients->count() . ' client(s) with no uuid.');
var_dump('Found ' . $admins->count() . ' admin(s) with no uuid.');

// Generate uuids for clients.
$clients->each(function (Client $client) {
    $client->uuid = Uuid::uuid4();
    $client->save();
    var_dump('Generated uuid ' . $client->uuid . ' for client id ' . $client->id);
});

// Generate uuids for admins.
$admins->each(function (Admin $admin) {
    $admin->uuid = Uuid::uuid4();
    $admin->save();
    var_dump('Generated uuid ' . $admin->uuid . ' for admin id ' . $admin->id);
});

Place in /admin directory and run while logged in as admin.