ORA-12638 Credential retrieval failed error is thrown when the authentication service fails to retrieve the required credentials of the user. ORA-12638 is a server side error and the user may not have too much control over resolving this error other than turning on the tracing on the client front. If you are unsure about how to troubleshoot the issue, please follow the steps mentioned below and provide the information to your database administrator.

ORA-12541 TNS no listener error is generated by oracle network layer. This error occurs when the listener for the oracle server you are trying to reach is down or unreachable. This error cannot be fixed by the user and should be resolved by the DBA. You can use the document and the links below to perform the basic troubleshooting before you open a ticket with the database administration team

DBMS_OUTPUT oracle package enables you to send output messages from various other PL/SQL program units like stored procedures, packages, triggers etc. DBMS_OUTPUT is primarily used for displaying messages or debug information from a PL/SQL Program. The procedures contained in the DBMS_OUTPUT package include DISABLE, ENABLE, PUT, PUT_LINE, GET_LINE, GET_LINES and NEW_LINE.

Oracle DATE datatype is used to store point-in-time values related to date and time. Oracle date datatype is stored in fixed length field of 7 bytes allows the storing of the year in 4 digit format, month, day, hours, minutes and seconds. Oracle dates are stored in julian era starting from January 1, 4712 BC through December 31, 4172 AD. Dates are defaulted to AD unless the format mask BC is specifically used

Oracle Rep-159 is an internal error generated by oracle reports. Oracle Rep-159 is generated in multiple situations which cannot be documented in detail, but a few of the situations that I have run into are listed below..

ORA-01555, Snapshot too old error in oracle indicates that something else other than your current process is creating the issue.  Oracle typically creates a read consistent view of the table so that it looks the same at the start and end of a sql operation. ORA-01555 indicates something has overwritten the undo area that keeps the data to maintain such a view.

ORA-01031 is generated by oracle in most cases when the user lacks privileges to perform the particular operation. In generic cases, ORA-01031 error is thrown when the user is unable to create an object like table, view procedure etc., due to an incorrect setup in the security layer. In some cases ORA-01031 can also happen during instance startup.

Oracle 11G introduces a new option to support compression of various structured, unstructured and other backup data. This option named Oracle Advanced Compression option provides multiple features including OLTP table compression ( this feature extends the  basic table compression that was available since 9i ) , Secure File Deduplication, RMAN Compression and Data Pump Compression. Oracle will continue to provide the basic Table compression in the Enterprise Edition of its product, but customers will have to license the new features along with Oracle 11G to get OLTP Table Compression and others.

ORA-00936, ORA-936 is a generic error indicating a missing expression in the SQL statement that you have just executed.  Most probably this error is indicating that you have either missed a , [ comma ] or the from clause or where clause or some other part of the SQL that has just been parsed.