I believe that the Family subscription would be for one version of Avast (Avast One I guess from your post), not different products as in your comment.
So I downloaded Avast Free and input the code from Avast One.
Theoretically you wouldn't need a subscription/license for Avast Antivirus Free for Windows, it just has to be registered.
So if you were using the Same Avast Product as the the one you have for the Family Subscription it should work. But not if it is for a different Avast Product such as Avast one and Avast Antivirus free version (which I use). They both have different components making up their Products.
This is all I could find relating to Avast One Family and would only relate to that product -
https://support.avast.com/en-gb/article/use-family-sharing/#pcImportant: Family Sharing is currently only available for the following subscriptions and devices:
Avast One Family: Family Sharing is available on Windows, Mac, Android, and iOS.
Avast Premium Security (Multi-Device): Family Sharing is available on Windows and Android.
Avast Cleanup (Multi-Device): Family Sharing is available on Windows only.
We plan to allow Family Sharing for more Avast subscriptions in the future.
Hopefully this will help, but essentially, you can only share your Family product within that same product. Avast One and Avast Antivirus are different products.