Avast Premium Security - Firewall

This has happened a few times, and I can’t find a way of fixing it.
Avast is showing that the Firewall is turned off.
When I click the option to turn it on, I get an error - “Firewall not available”

Any ideas on how to fix this problem?

https://support.avast.com/en-eu/article/Repair-Antivirus

Thanks.
I will try that.