If you have not activated the Device Administrator mode yet, you can do this in the settings of Avira Free Android Security.
Please follow these steps to activate the Device administrator:
Once the device administrator is enabled, you do not have any possibility to uninstall Avira Free Android Security from your device. You need to disable the device administrator first in order to uninstall the app. For further information regarding the uninstallation of the app please see following article.