It is the core responsibility of the Viber, to maintain the security and privacy of the user. Therefore, to ensure that the chats and conversation are encrypted you find the lock sign during your chat.
Now the question is what to do if that sign is missing. In that situation first of all check your Viber status from Play Store. Make sure you have the latest version of Viber that supports the chat encryption feature, if your phone has an old version, than immediately update your Viber app.
If your app is updated then ask group members to check their version. In this way, you and your group members have an updated version and definitely the sign will appear on your group chat.