ORA-12154 TNS: could not resolve the connect identifier specified with SQLPLUS
So you don't need to specify the full path when you run sqlplus.exe:

Set your %TNS_ADMIN% variable to  the full path of the relevant folder (C:\programs\Oracle\Ora11g\TNS_ADMIN ?). Instructions for doing so are a google away.  At the same time, make sure %ORACLE_HOME% is set to C:\programs\Oracle\Ora11g and that your %PATH% has C:\programs\Oracle\Ora11g\bin added to it.

The problem you're actually having is because of the parameters you're passing to sqlplus.

You've been doing:

C:\programs\Oracle\Ora11g\BIN\sqlplus username@schema_name


Which is wrong &amp; interpreted by sqlplus as:

C:\programs\Oracle\Ora11g\BIN\sqlplus username@database_name


Also, in Oracle, the owner of a schema (hence the schema name) is the user name. Obviously, grants can be given to other users to perform DML/DDL on other schemas, but you still have to login with a user with sufficient rights. (Proxy logins exist, as does alter session set current_schema=blah, but both are beyond the scope of this answer).