Errors - Database Links in Oracle

ORA-00987: missing or invalid username(s)

Cause:

No username was specified in a GRANT statement or one of the specified usernames is invalid. Valid usernames must be specified following the keyword TO in a GRANT statement to define a user. A username must begin with a letter, consist only of alphanumeric characters and the special characters $, _, and #, and be less than or equal to 30 characters. If it contains other characters, it must be enclosed in double quotation marks. It may not be a reserved word.

Action:

1. Specify a valid username, or list of usernames, following the keyword TO in the GRANT statement.
2. When this error Occurs then check the Service Name is added in the tnsnames.ora File in your system path.

ORA-12154: TNS:could not resolve service name
This is because the value of INIT.ORA must be set TRUE.

Error on line 0
Error message: "ORA-12154: TNS: could not resolve service name"

Verify that the host or database name is correct and that it is listed in the TNSNAMES.ORA file.
TNSNAMES.ORA file,this is a file stored locally on your computer in the Network --> Admin folder inside your Oracle home folder. This file keeps a list of available databases and related information required to connect to them.

ORA-01017: invalid username/password; logon denied
ORA-02063: preceding line from ..

This is because the Parameter - GLOBAL_NAMES = TRUE, Set as GLOBAL_NAMES = FALSE by Alter Session command to resolve the error.

ORA-28030: Server encountered problems accessing LDAP directory service

Yes. This was indeed the case using "current_user" to create the link.Drop and re-create the link using the name of a user account, it will work fine.