You don't say what your current Avast program version and build number is ?
What or why can't you download e.g. what errors are you getting ?
Have you tried a system restart ?
I have my Avast Program set to manual program updates as I monitor the Avast forums to see when a new program is released, what changes have been made, any bug fixes are in it, etc. If I don't feel the changes or bug fixes effect my system, I just keep monitoring and see if there are any reported issues with the update.
You have posted in the beta sub-forum, but I assume (dangerous I know) you aren't on the Avast Beta stream.