I bot 20 dollar psn card and I can't buy a 20 dollar game like minecraft it tells me to redeem more money?

Sony is required (by state laws) to collect sales tax on games in some areas, typically where the "Brick & Mortar" rule applies (as they have a physical presence in that state, like California) OR there are "Amazon Tax" laws being enforced (like New York).

The sales tax gets added on the back-end prior to checkout & payment, which pushes the final price of the game OVER the noted sticker price. While you may be able to change your zip code to avoid paying sales tax (by claiming you're in a state that does enforce sales tax on online transactions), you may run into potential problems with IP geo-checking (where the Playstation Store checks the reported location of the IP address used) & using credit cards (as you are required to match the billing zip code for your card).

While a little research may be required for the exact reason (for future reference), it's an unfortunate thing that you'll have to deal with.