firewall won't turn on

I can’t turn my firewall on. A pop-up comes up and says it isn’t available.
I also have warning at the top of my summary page that says: “Your system is not fully protected. Please use fix now button to start fully protecting your system.” But that button won’t work.
I am using 6.0

What firewall ?
What version of avast are you using, Free/Pro/AIS ?
What operating system ?

Try a repair of avast:
XP - Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow.

Vista, win7 - Control Panel, Programs & Features, uninstall a program, select ‘avast! Anti-Virus,’ click the Change/Remove button and scroll down to Repair, click next and follow.

You may need to reboot after the repair.

https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=1050