One who does not know how to save money

One word substitute is Spendthrift.

Reckless: heedless of danger or the consequences of one's actions; rash or impetuous.
Lavish: sumptuously rich, elaborate, or luxurious.
Careless: not giving sufficient attention or thought to avoiding harm or errors.
Spendthrift: a person who spends money in an extravagant, irresponsible way.