iCloud Back-up
Why am I not able to back-up my phone (family sharing) despite my iCloud having 1.76 TB still available?
iPhone 11 Pro Max
There are a few suggestions in here, but I don’t know how useful they will be.
If you can't back up to iCloud or restore from iCloud backup failed - Apple Support
I would try the basics, try another WiFi network, restart your device, force restart your device, power cycle your router or any combination of those things.