I am seeing the error 'Your serial number is not valid’ when attempting to activate through the Unity Hub. What does this mean?

Symptoms:

• I would like to activate my license via the Unity Hub, however, I am receiving errors.
• I am seeing the error message ‘Your serial number is invalid’ when attempting to use Unity.
• I am seeing the error message ‘Your serial number is not valid’ when attempting to use Unity
• I would like to access the Unity Editor, but I am receiving errors in the Hub.
• The Hub is saying my license is invalid.

Cause:

The most common cause for this error is the license key running out of activations. If you have recently updated your OS or changed any hardware, this can alter the hardware ID to which the Unity License File (.ULF) is attached, effectively making the current .ULF redundent. 

Another cause for the error 'Your serial number is not valid' is a connection issue between your machine and the Unity license servers. 

Resolution:

You can return activations made in Unity Editor versions 4.5 and higher via your account page. To do this see the article ‘How do I return the activations on my Pro/ Plus/ Enterprise license?’.

If after completing the above process you still receive an error, then it is likely a connection issue between your machine and our license servers. In which case, please check any firewall/ proxy settings, whitelist URLs and disable any antivirus software.

Failing that, manually activating would be the next process to follow: https://docs.unity3d.com/Manual/ManualActivationGuide.html 

If releasing your activations & checking your connection still does not help you to activate your license, you may be experiencing a conflict with the license file saved on your machine. You will need to delete the existing license files and create a new one:

1. Delete any files found here: (If the files are hidden, you need to click View and tick the Hidden Items check box.)
   - Windows: C:\ProgramData\Unity
   - Mac: Library/Application Support/Unity
   
   
2. Restart your machine (do not skip this step)
   
   
3. Reactivate your license key following the steps in the article ‘How do I activate my license’

If your license is still not activating after troubleshooting the above, please contact Customer Service with your Hub log file so we can take a closer look at your issue. 

More information:

I have been asked to provide a log file, where do I find this?
How do I activate my license?
Manual activation guide
How do I return the activations on my Pro/ Plus/ Enterprise license?