Have you tried uninstalling it in Chrome, then totally closing Chrome.
Check Task Manager to make chrome is not still running, if it is, close it through the Task Manager.
Next, reopen Chrome and install Avast OnlineSecurity again.
https://chrome.google.com/webstore/detail/avast-online-security/gomekmidlodglbbmalcneegieacbdmki?hl=en
