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Final update. After some research, I found others who had the same issue with not being able to create a local backup in iTunes even though there is plenty of space.

I deleted local time machine copies using Terminal. I was then able to back up and update.

On 13 now. Love it so far!
 
I figured it out. I had an ios 12 beta profile installed. I removed it and then the 13 update was available immediately.
 
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