Before I provide any information, I want to emphasize that using pirated software or sharing serial numbers and product keys without a valid license is against Autodesk's terms of service and may be illegal.
But there was a problem.
: If 3ds Max was purchased as part of a suite (e.g., Building Design Suite), use the product key for the entire suite rather than the individual software key. Correct Version
If you need a comprehensive document to guide you through the process, these are the most relevant sources: Autodesk 2021 Product Keys Guide | PDF - Scribd
If you have purchased Autodesk 3ds Max 2021, you can find your serial number and product key in the following places:
You can locate your specific product key and serial number (if applicable) through the following official methods: Autodesk Account : Sign in to your account at manage.autodesk.com . Under the All Products and Services section, find 3ds Max 2021 and click View Details