I will explain TNS_ADMIN Environment Variable in Oracle in this post.
TNS_ADMIN in Oracle
TNS_ADMIN environment variable is very important in Oracle software that points to the directory where SQL*Net files like tnsnames.ora, sqlnet.ora and listener.ora.
TNS_ADMIN points to the $ORACLE_HOME/network/admin in Linux/Unix and %ORACLE HOME%\network\admin directory in Windows environment.
You can set it in Linux as follows.
export TNS_ADMIN=/u01/app/product/12.2.0/dbhome_1/network/admin
Or You can set it in Windows as follows.
set TNS_ADMIN=C:\app\oracle\product\12.2.0\dbhome_1\network\admin
TNS_ADMIN in Oracle Software Registry for Windows
You should run regedit to open a Registry Editor and then go to following directory.
You will find the TNS_ADMIN variable, if not exist or it is incorrect, you can edit it.
Computer\HKEY_LOCAL_MACHINE\SOFTWARE\ORACLE\KEY_HOME_NAME
Do you want to learn Oracle Database for Beginners, then read the following articles.
https://ittutorial.org/oracle-database-19c-tutorials-for-beginners/