Network cables are not connected properly.  Check network connections.
Real-time computer is not turned on.  Confirm that the real-time computer is turned on.
Real-time computer settings are incorrect. See Section 3.2 – “Configuring the Real-Time Computer (xPC Target boot disk)”.
Network IP address for GUI computer is set incorrectly. See Section 2.6 – “Connecting the Computers (Dedicated LAN)”.
xPC version incompatibility. The real-time computer needs to boot to the same xPC version that the Task Programs are compiled with. See Section 11.4 – “xPC Target Version Configuration”.