Hello externalreality,
The first thing we would suggest would be to power cycle the iPhone by following these steps found at Restart your iPhone - Apple Support:
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, Force restart iPhone - Apple Support
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
While simple, this can refresh the iOS, apps and connections while resolving some issues.
Next, we would check and confirm the available iCloud space to confirm there is room. Tap on Settings > [your name] > iCloud. From here you can see how much iCloud space is being used, along with how much is available.
Also confirm the size of the backup. Tap on Settings > [your name] > iCloud > Backups < [device name]. You can see the backup size here.
If it looks like there is enough space, then we need to look for other reasons that the backup is not happening.
In order for the Automatic backup to occur, the iPhone needs to be connected power, locked, and connected to Wi-Fi. More information is available here: Back up iPhone - Apple Support
Be sure that all of these conditions are met.
Have a good day.
