welcome to the forum. could you post a picture of that error please.
any third party firewall that is blocking avast from updating?
like comodo, outpost?
any other strange behaviour on your computer?
could be malware related.
do a scan with malwarebytes antimalware for a second opion
http://filehippo.com/download_malwarebytes_anti_malware/
download install update and do a scan, and don’t forget to remove what it finds. a system reboot might be needed.
good luck and let us know on the progress or if you need more support.