Iphone 7 iCloud backup not working

I'm having a storage issue on my iPhone 7 with iOS 13.5. I made a prior post about it, linked here, and received a recommendation to do an iCloud backup before updating to the latest iOS and restoring. Unfortunately I keep getting a message saying "The last backup could not be completed." I don't currently have a computer I can backup to. Any thoughts on what might be causing this? I am currently not using any of my iCloud storage.

Thanks!