I had this issue for the last 10 days. Today it hit expired so I started trying different things.
I found a way to fix it:
Under subscription window (pictured above) you can click on the little dark grey arrow on the right beside “Avast Free Antivirus”.
Clicking this will display your activation key.
Copy it (Ctrl+C), then click “insert activation code” and paste (Ctrl+V) into the box.
Hit activate.