Problem

When opening Exor forms this error appears, ORA-12154: TNS: could not resolve the connect identifier specified. 

Solution

Check if this issue happens for single user or multiple users.

Steps to resolve/fix ORA-12154 :

1. Does TNSNAMES.ORA file exists and is in the proper directory and is accessible$ORACLE_HOME/network/admin/tnsnames.ora
2. Is there any syntax errors anywhere in the TNSNAMES.ORA file? Unmatched parentheses or stray characters for example?  All parenthesis around the TNS entry should be properly terminated. There should be no control characters at the end of the instance or database name
3. Is service name used as the connect identifier exists in the TNSNAMES.ORA file
4. Check instance name has been entered correctly in tnsnames.ora.
5. Make sure that "TNSNAMES" is listed as one of the values of the NAMES.DIRECTORY_PATH parameter in the Oracle Net profile (SQLNET.ORA)
6. If problem still persists, try to re-create TNS entry in tnsnames.ora.
7. Domain name entry in sqlnet.ora should not be conflicting with full database name.