SP2-0750: You may need to set ORACLE_HOME to your Oracle software directory

This error caused by not specifying your ORACLE_HOME in environment variables.
In Unix/Linux
su -oracle
pswd

ORACLE_HOME=/u01/app/oracle/product/db_1 (or like your path to your Oracle home)
export ORACLE

(also, make sure you have given your ORACLE_SID)

In Windows
in Command Line

set ORACLE_HOME=C:\.....