What Avast free program version number ? 18.3.2333
How long is left on your existing registration (not a license on avast free) ? It says it's expired.
How are you trying to renew license ? From it's main screen, from the "warning" popup. It all takes me to a choice screen between two options: Free and $20 upgrade, but only the Upgrade button is highlighted and leads to anything. The "free" option just closes the screen and when it comes back, it's back to a warning screen of expiration, or the regular main screen with a warning in it.
Why/How is the program stopping it, e.g. any error messages displayed ? Nope, not a one, just won't process the "free" option. I read somewhere I might have to reinstall it, but haven't tried yet.