The activation of Avast Free is affecting a lot of users. Avast are aware of it, but first attempt at fixing it seems not to work for some people.
This post Unable to reactivate Avast Free antivirus - #50 by 111jiri.kubala suggests uninstalling Avast using their uninstall program avastclear.exe that you can download from Avast. Avast Removal Tool | Download Avast Clear | Avast
Then install Avast Free again using the offline installer e.g. from this post. New version 25.8
I do not know the success rate of this.
Note I am not an Avast Team member, just another user.