Connect to db first ! That's causing the error-message.
There is also a logical error in your query; don't include to_char in text-literal:
select decode(:P_TransactionType, 'Remove from UPA', 'UPA Details Removed by Transaction Code'
, 'Deposit to UPA', 'Deposit to UPA Summary for '||to_char(:P_Effective_Date, 'mm/dd/yyyy'))
The way you code it, the output would not include a date, but the literal [[Deposit to UPA Summary for ||to_char(:P_Effective_Date, "mm/dd/yyyy")]]
See the difference ?