For what's it worth, I have the exact same problem.
- windows is fully updated,
- avast is fully updated,
- uninstalled avast fully, then reinstalled,
- no firewall selectable in windows security,
No matter what I do avast firewall will not turn on. At this point I'm unsure if it was an avast update or a windows update that did it in. But here I am paying for avast premium security and browsing the internet without a firewall.