Hello ayten16,
Thanks for reaching out through the Apple Support Communities about that difficulty with setting up iCloud on your new iPhone 12 mini.
To verify, are you being prompted to provide the passcode used to unlock your prior iPhone, or a 6-digit verification code?
If you are being prompted for the passcode for your prior iPhone, do you still have your prior iPhone? And if so, are you able to unlock it using your prior 4-digit passcode?
If you are being prompted for a 6-digit verification code, you may be able to get that code from the Settings of a trusted device: Get a verification code and sign in with two-factor authentication
Hopefully those details and resources will help to further clarifying and address your situation.
Take care.