subscriptions use "Sign In" activation and may not require a serial number at all. Autodesk Community, Autodesk Forums, Autodesk Forum
: If you already have the software installed, you can find the serial number by typing _PKSER into the AutoCAD command line. Serial Number For Autocad 2021 Product Key 001m1
If AutoCAD was purchased as part of a suite, the key changes (e.g., 768M1 for AutoCAD Design Suite Premium 2021). Cadac Group 2. How to Retrieve Your Serial Number Autocad 2021 Product Key and Serial Number subscriptions