Why does my iphone 7 cancel the backup to icloud when I have the space, power, and a solid internet connection.

I am trying to backup my iphone 7 because I have to send the phone in for repair on some other issue. However, each time I go to back it up, it gets cancelled. icloud made me purchase 50 GB for .99 / month and that wasn't enough, so then it came back and wanted to add another 200 GB for $2.99 / month on top of what I already paid. I found out I only needed 70 GB but apple / icloud doesn't tell you that.

Anyhow, I bought the 200 GB, started the backup, walked away after watching it for 20 minutes (and watching the little line move so it was doing something), then came back 2 hours later and it said the backup failed for some reason. I have great wifi, my phone was plugged in to my computer with solid power, with the computer NOT going to sleep. I checked my phone and sure enough it did not get backed up. It said last backup: NEVER.

Anyone know what to do here?