Why does XRP have a minimum balance? (Why I cannot empty out my XRP account?)

Ripple's XRP is a special asset that has built-in rules on minimum balances held in XRP addresses, just like some bank accounts where you have to maintain a minimum balance.

In order to activate and use XRP, you must first deposit no less than 20 XRP into your ELLIPAL Ripple address.

After that, your account will activate and everything will be just like you are used to with Bitcoin, Ethereum or any other asset, except that the 20 XRP minimum cannot be moved out.