This is a warning to device end-user that it cannot find a valid and secure certificate bind to 192.168.100.51 for connection to you MobiControl server.  You can just press "yes' and the agent will continue and likely communicate successfully with your server.  However, such warning  may pop up again the next time it try to connect to your server .

BTW, unless your devices and Mobicontrol server are intentionally kept in a closed internal network (e.g. corporate network in office premises, virtual private mobile phone network associated with special APN,  etc), it is very unusual to use private address 192.168.100.51 for your server.  If really so, you just need to update your server certificates with MCadmin.exe.  Please contact Soti support team to get details on what to do.

On the contrary, if your devices are meant to be connected to the server via public network on the internet,  your Mobicontrol server should be configured to use unique public IP address/FQDN (Fully Qualified Domain Name).   The corresponding server certificates should also be updated.