I have this problem too on my Asus Win10 laptop since this month (early Oct), can not update virus definition, it seems like Avast is unable to connect to the internet.
What I have done so far:
1. Check the internet connection = internet is working properly, browser and download is normal
2. Connect to another network = fail
3. Disable all firewall = fail
4. Repair Avast = fail
5. Uninstall and Reinstall Avast = fail, and i can not install Avast properly afterward, always stop in the middle of installation process, Avast is shown on Processes at Task Manager, but there is no Avast Icon and Folder on start menu and taskbar, like an incomplete installation.
6. Uninstall Avast using Avast Removal Tool (avastclear.exe) on safe mode and Reinstall Avast = fail, same result as no.5
7. Uninstall Avast and try another Anti Virus (Avira) = Avira is installed properly but some menu is missing and unable to update also, like Avast it's like Avira is unable to connect to the internet
8. Uninstall Avira and Reinstall Avast using Offline installer = Avast is installed properly but still cannot update virus definition, I can perform manual virus definition update using Avast offline virus definition updater on this page >
https://www.avast.com/en-id/download-update#pc(Fail = Can't update virus definition)
I suspect there is some virus or malware on my computer that prevent any antivirus to update it's database.
Please help, thank you.