I have received the below error when I tried using sqlplus on IBM AIX server with Oracle 12c installed on it and upgraded:

sqlplus ‘/as sysdba’

exec(): 0509-036 Cannot load program sqlplus because of the following errors:

rtld: 0712-001 Symbol CreateIoCompletionPort was referenced

      from module /oracle/oradb/product/12.1.0/lib/libttsh12.so(), but a runtime definition

      of the symbol was not found.

rtld: 0712-001 Symbol GetMultipleCompletionStatus was referenced

      from module /oraclex/oradbp07/product/12.1.0/lib/libttsh12.so(), but a runtime definition

      of the symbol was not found.

rtld: 0712-002 fatal error: exiting.

Solution

Change the IOCP IBM AIX parameter from “defined” to “available”, this action requires root privilege

# smitty iocp
Select Change / Show Characteristics of I/O Completion Ports.
Change configured state at system restart from Defined to Available

Remark: it is a prerequisite to change IOCP parameter before upgrading the database to 12c in IBM AIX environment.

I hope this helps………