Oracle/PLSQL: ORA-00904 Error

Error:	ORA-00904: invalid column name
Cause:	If you trying to execute an SQL statement that included an invalid column name or the column name is missing.
Action:	The options to resolve this Oracle error are: Rewrite your SQL to include a valid column name. To be a valid column name the following criteria must be met: The column name must begin with a letter. The column name can not be longer than 30 characters. The column name must be made up of alphanumeric characters or the following special characters: $, _, and #. If the column name uses any other characters, it must be enclosed in double quotation marks. The column name can not be a reserved word.