The Three Criteria for Effective taxes are: 1) equity - taxes should be impartial and just 2) simplicity - tax laws should be written so that both taxpayers and tax collectors can understand them 3) efficiency - tax should be relatively easy to administer and reasonably successful at generating revenue.
Flat tax is a single tax rate imposed on individuals regardless of income. It is not equitable because there are those who earn minimum wage but are still subjected to tax. It would lessen their take home pay.
Since Flat Tax rate already failed the equitable criteria, it can't be an effective tax.
