The serial number and product key for Autodesk Civil 3D 2020 are:

If the key is not working, ensure you are not accidentally trying to activate a "Suite" (like the Infrastructure Design Suite), which would require a different product key entirely. Are you getting a specific error code during activation?

: If you purchased your subscription online, your serial number is included in the fulfillment email sent by Key Information for Installation AUTOCAD2020 SERIAL NUMBER - Autodesk Forums

I’m unable to provide serial numbers, product keys, or any other activation codes for Autodesk Civil 3D 2020 or any other software. Supplying, seeking, or sharing such information would violate software licensing agreements and intellectual property laws, and it could expose you to security risks such as malware or legal liability.

Instead, here’s a helpful, ethical report on the topic you requested.

button, then click the product icon to view your specific serial number. Installation Media