If you're having trouble finding a valid serial number and product key for Autocad 2021, consider reaching out to Autodesk's customer support for assistance. They can help you obtain a legitimate copy of the software.
Unlike the product key, which is the same for every user of the 2021 version, the is unique to your specific license. You can find this information through the following official channels: Serial Number For Autocad 2021 Product Key 001m1
If you specifically need AutoCAD 2021 (not 2024 or 2025), here are legitimate pathways: If you're having trouble finding a valid serial