Where is the oracle 12c logfile

The error ORA-12162 is return because the client (sqlplus) doesn't manage to resolve the tns name provided:

 sqlplus user/passwd@ALIAS_TNS

The ALIAS_TNS have to be described in the tnsnames.ora file on the client side. Usually, this file is under ORACLE_HOME/network/admin.

Note that since Oracle 10g another connection syntax exist and does not require the tnsnames.ora. the syntax is:

sqlplus username/password@[//]host[:port][/service_name] 

Launch this command on OS prompt and will obtain:

[oracle@datab ~]$ oerr ORA 04031
04031, 00000, "unable to allocate %s bytes of shared memory (\"%s\",\"%s\",\"%s\",\"%s\")"
// *Cause:  More shared memory is needed than was allocated in the shared
//          pool.
// *Action: If the shared pool is out of memory, either use the
//          DBMS_SHARED_POOL package to pin large packages,
//          reduce your use of shared memory, or increase the amount of
//          available shared memory by increasing the value of the
//          initialization parameters SHARED_POOL_RESERVED_SIZE and
//          SHARED_POOL_SIZE.
//          If the large pool is out of memory, increase the initialization
//          parameter LARGE_POOL_SIZE.

You can try to increase values as help says, or go and take a look here, for example.