Oracle Package in Data Dictionary that can't be dropped
I am suspecting with the case sensitive name or special character(i.e. ' or white-space) included with the package name unknowingly while creating package. I tried to get object name from dictionary itself which ensures correct object name in your drop command in the following script, it can help in your context:

select 
'DROP PACKAGE ' || '"' || OBJECT_NAME || '";' as "Command" 
from 
USER_OBJECTS 
WHERE 
OBJECT_NAME LIKE '%POSTING_PROCS%'
AND OBJECT_TYPE='PACKAGE';


CAUTION - REMEMBER THAT ABOVE SCRIPT GENERATES DROP COMMANDS WHICH CAN CONTAINS POSTING_PROCS AS THE PART OF OTHER PACKAGE NAME, PLEASE CHECK EACH DROP STATEMENTS BEFORE EXECUTING THEM IN YOUR SERVER.

Alternatively - You might be using client tool like TOAD, SQL Tool, SQL Developer which display your invalid object name in a GUI format. You can try to delete your object by right click on the object and follow with the drop option.