This blog is more about information than individual user support.
You’ll find support ressources here http://www.avira.com/en/support
To (maybe) shorten your way to a solution …
Check if your Avira product is up to date. Avira is update compliant and sets the necessary registry entry expected by Microsoft to show and do the update.
See Microsoft support information about Windows 8.1 security-only-update KB4056898
https://support.microsoft.com/de-de/help/4056898/windows-81-update-kb4056898
This document contains the information you need to proceed.
– how to determine whether your system is compliant and ready for this update
– how to obtain detailed file information to check if your system is already up to date
– how to download the update manually if your Windows Update can’t for whatever reason.
Please note:
1. The update might not show up because it is already installed. Check your update history in Windows Update if kb4056898 (available since Jan. 3, 2018) is done… most likely if automated install and restart are activated.
2. If installation shows up as having failed, this might be incorrect (known issue on windows 10) and might be verified by Microsoft Windows build number or by comparison of the versions of the files delivered by this update.
3. I’m just a user, too, as you are. I’m no frequent visitor here, just came along reading your problem and trying to help you sort it out. Don’t expect me to come back soon…^^