Keep getting asked to add device as trusted device

Trusted devices

A trusted device is an iPhone, iPad, or iPod touch with iOS 9 and later, or Mac with OS X El Capitan and later that you've already signed in to using two-factor authentication. It’s a device we know is yours and that can be used to verify your identity by displaying a verification code from Apple when you sign in on a different device or browser. An Apple Watch with watchOS 6 or later can receive verification codes when you sign in with your Apple ID, but cannot act as a trusted device for password resets. 

Trusted phone numbers

A trusted phone number is a number that can be used to receive verification codes by text message or automated phone call. You must verify at least one trusted phone number to enroll in two-factor authentication.

You should also consider verifying an additional phone number you can access, such as a home phone, or a number used by a family member or close friend. You can use this number if you temporarily can't access your primary number or your own devices.

Verification codes

A verification code is a temporary code sent to your trusted device or phone number when you sign in to a new device or browser with your Apple ID. You can also get a verification code from Settings on your trusted device.

A verification code is different from the device passcode you enter to unlock your iPhone, iPad, or iPod touch.