I can not set my passcode? How come?

To unlock your iPhone, follow the steps described in the Apple article:

(< iOS 15.2) ->: If you forgot your iPhone passcode

If you forgot your iPhone passcode - Apple Support

(iOS 15.2 or iPadOS 15.2 and newer) If you see an iPhone or iPad unavailable message or Security Lockout screen

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

If you did not set up the passcode, then someone else who was using your phone did. Think about who that might be, and then ask them for the passcode so you won't be locked out of your own phone.