Backup
How to connect power lock and Wi-Fi during backup
iPhone 6 Plus, iOS 12
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If you want the iPhone to automatically backup to iCloud, it needs to be connected to wi-fi as described in Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support
and the phone needs to be locked. You can lock the phone by pressing the right side button or you can just let it lock itself based on the time choses in Settings > Display & Brightness > Auto-Lock.
If you are using the iCloud Backup Now option, the phone needs to be connected to wi-fi but not locked.
Also see: How to back up your iPhone or iPad with iCloud - Apple Support
An exception to the wi-fi requirement is if you have an iPhone that supports iOS 14 or later (your iPhone 6 Plus does not) and a carrier that allows backup over 5G cellular. See: Back up iPhone - Apple Support (ZA)