Review the error information, check that the connection parameters are correct, and try to connect again. For more information, see Connecting to a system from the Host Connections view.
If you are using CICS Explorer and connecting to a CICS system, and specifically to a CICS® TS v4 system, and the connection failed with the error NOTAVAILABLE CPSMAPI, it might be that you did not include the SEYUAUTH and SEYULOAD libraries in the STEPLIB & DFHRPL JCL respectively, or if you are connecting to a stand-alone (SMSS) CICS Region, there is an inconsistency in the version levels of the libraries. For more information, see Setting up the CICS management client interface.
If you are using CICS Explorer and connecting to a CICS TS for VSE/ESA 1.1.1 system, check that the DEFINE PROGRAM, DEFINE TRANSACTION, and DEFINE DOCTEMPLATE for CICS TS are correct.