You can get only user information that users agree to provide. When users log in with Kakao, they can choose what to provide to your service by consent items on Consent screen. Then, Kakao provides you with the personal information that users agree to provide through User management API.
Configure consent items needed for your service in [My Application] > [Product Settings] > [Consent items].
You cannot get the user information if users refuse to provide even though it is set as ‘optional consent(선택 동의)’ and if it is set as ‘Not Use(사용 안함)’.
To set some user information as ‘required consent(필수 동의)’,
you need to apply for KakaoSync and get approved OR switch your app to a Biz app by registering the Taxpayer identification number(TIN)/Business registration number of service in [My Application] > [App Settings] > [Biz app].
Refer to User Information to see what process is needed by user information.