Why isn’t my phone automatically being backed up

In: How to back up your iPhone, iPad, and iPod touch

https://support.apple.com/HT203977

see the section: "Automatically back up with iCloud Backup

To let iCloud automatically back up your device each day, here's what you need to do:

Make sure that iCloud Backup is turned on in Settings > [your name] > iCloud > iCloud Backup.

Connect your device to a power source.

Connect your device to a Wi-Fi network.

Make sure that your device's screen is locked.

Check that you have enough available space in iCloud for the backup."