You bought a used phone without a receipt, you live in a country without an Apple Store, or you cannot prove the phone is yours.
When you remove a screen lock via software on an unrooted device, you are essentially telling the system to overwrite the encryption key. Because the system cannot decrypt the user data partition without the password, it erases it. dr unlocker
However, you must set realistic expectations. You will likely lose your data. You will likely pay $40 to $50. But you will regain a $500+ device. You bought a used phone without a receipt,