Sometimes it does not work in remote sessions or it needs an elevated command prompt.
Did you have a look inside the Setup and Application event logs?
Usually you have some information there about software installation and uninstallation.
Not having worked with its uninstaller, I’d have to ask if the uninstaller requires user input. Uninstall strings aren’t always automatic and many times requires additional user input or additional command line parameters. Easiest way to determine this is to just open up an administrative shell and execute whatever is in the uninstall key. If it requires you to move forward, you need to figure out what additional parameters it needs.
On Windows 10 you can do:
Get-Package -Name "*Avira*" | Uninstall-Package
Again, only works if the uninstall string doesn’t need additional parameters to automate the uninstaller.