This error occurs because either the IBM Communications Server or the CA TCPaccess Communications Server needs to be specified.
TCP/IP sockets interface is started by the $NM SOCKETS parameter group in Customizer.
Enter /parms to access the Customizer parameter groups and check that the $NM SOCKETS has the correct value for TCP/IP Software Type specified. The available options are IBM, IBMV6 or TCPACCES. Once a value has been chosen, the parameter group must be actioned.