Unknow Icloud username

Are you sure that you purchased the phone as new, or did you buy it from someone else?

If you purchased it from someone else, then it is likely that their Apple ID is linked to iCloud and you need to contact them to get them to remove it, or you will never be able to use the phone as you have come up against the Activation Lock that is an anti-theft device.

There maybe two different but related things here. An Apple ID is used to access Apple's iCloud service, and Apple ID is also used to access the App Store and the iTunes Store, it is possible to use the same Apple ID for both or use different ones.

First to check to see if your iPhone currently has an iCloud account configured go to Settings -> iCloud. If a name and email address is listed under iCloud then this suggests your iPhone is logged in to iCloud. If that name and account is not yours but the sellers then only they can logout of the account and enable you to if you wish login using your own account. (iCloud is free.)

While iCloud offers a variety of services the most important one is 'Find my iPhone' this of course allows you to find your missing iPhone but also prevents someone else from wiping your phone if they find it. If it is logged in as the previous owner then you will not be able to wipe it. You might also be able to check if your phone is locked to an iCloud account via this page https://www.icloud.com/activationlock/

Apple may be simply trying to encourage you to setup iCloud.

The second possibility is that when you connect the iPhone to your computer and iTunes it is trying to sync data between the two devices particularly either Apps or music/movies purchases. In order to do this the iPhone and iTunes need to be logged in to the same account i.e. Apple ID. This might therefore prompt a message.

Bahruzpasha wrote:

Icloud username is not mine and the sellers.I am using this phone no any problem.I create my ID and I know my ID password.But when i check icloud setting I see another user email and name and surname.

If there is someone else's iCloud account already listed then the phone was clearly used by someone else previously. Either this means it is thirdhand or it means the seller has possibly sold you a stolen/lost phone. As I mentioned only the person who has access to that account can log out the phone from that account, this is part of the 'Find my iPhone' security.

You could ask the seller where they got it from and to put you in touch with the person whose account that is - unlikely if it is lost/stolen or try emailing that address directly. Apple will not be able to unlock it from that account, only the owner of that account can do this.

See Find My iPhone Activation Lock - Apple Support