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.