How do I lock my screen to back up my iPhone 7
please explain how to "Lock" the screen? to back up to iCloud
iPhone 7 Plus, iOS 12
Just make sure your device is not turned off, and is sleeping.
According to: Access features from the iPhone Lock screen - Apple Support
'The Lock screen, which shows the current time and date and your most recent notifications, appears when you turn on or wake iPhone. From the Lock screen, you can see notifications, open Camera and Control Center, get information from your favorite apps at a glance, and more.'