I've had a similar problem on one of my two laptops, so my experience might be useful to you?
Both laptops are running Windows 10 64-bit, v1803. Avast Free. One had received the August Avast program update and was still working correctly. The other hadn't received it, and was then saying that it hadn't received virus definition updates for a few days. I tried a manual program update, but it just stuck on "Initialising".
I'm currently on mobile data, with each laptop set to see the network as "Metered". I don't think this makes any difference - Avast seems to ignore that setting and has always been able to update previously. When I was using Windows Defender recently, that never seemed to update itself when on a metered connection. (I didn't try it unmetered.)
After rebooting, and trying several times to get it to update, I gave up and tried to uninstall Avast. That couldn't be done as it was already trying to update.
I tried using the Avast Clear program, without booting into Safe Mode. It just kept moving a bar back and forth for an hour but did nothing. Had to force a reboot, and then google to find how to get into Safe Mode, as Microsoft have disabled the f8 method.
After properly cleaning Avast off the laptop, I found that Windows Defender didn't want to start up, so I needed to get Avast going again. I had an off-line installer for the previous (June?) program version, so I installed that successfully. (Didn't want to pull in 250MB again for a new installer.)
Manual definitions updates worked fine, and I was also able to manually upgrade to the current program version. Task Manager seemed to indicate that Avast had done an Emergency update during the upgrade.
All seems to be working OK now, though it hasn't been going long enough to see whether automatic updates are working correctly.
One other thing - my wife had alerted me to the problem initially because she'd seen an Avast popup warning about some spyware, but the popup had vanished by the time I got to it, and there seems to be no way to recall earlier warnings - at least not this type of warning. I've now set popups to stay on screen for 500 secs instead of 20 secs.
Hope that helps.
