This error message pops when your account has been logged in on another device. If it was not done by you and you suspect someone else was using your account, simply go to Settings> Basic> Logout> Logout from all devices.
Take note: if you share your login credential with someone else, there is no guarantee that your account would not be hacked.
Therefore, it is important to protect your login information. For more details on how to protect your account please refer to this article: How to Protect Account.