If you have not activated the Device Administrator mode yet, you can do so in the settings of Avira Antivirus Security.
Please follow these steps to activate the Device Administrator:
Once the Device Administrator is enabled, you cannot uninstall Avira Antivirus Security from your device. You need to disable the Device Administrator first in order to uninstall the app. For further information regarding uninstallation of the app, please see our article Uninstallation of Avira Antivirus Security not possible.